Tybost (Cobicistat): Uses, Dosage & Side Effects
Pharmacokinetic enhancer (booster) for HIV protease inhibitor therapy
Tybost (cobicistat) is a pharmacokinetic enhancer used as part of combination therapy for HIV-1 infection in adults and adolescents aged 12 years and older. It does not fight HIV directly but acts as a booster for the protease inhibitors atazanavir or darunavir, increasing their blood levels so they remain effective for longer. Tybost must always be taken with food and in combination with either atazanavir or darunavir as part of a complete antiretroviral treatment regimen.
Quick Facts: Tybost
Key Takeaways
- Tybost (cobicistat) is a pharmacokinetic booster with no direct antiviral activity — it works by increasing blood levels of atazanavir or darunavir
- Must always be taken with food and in combination with either atazanavir (300 mg) or darunavir (800 mg)
- Has extensive drug interactions due to CYP3A inhibition — numerous medications are contraindicated
- Not recommended during pregnancy as blood levels may decrease, reducing the effectiveness of the boosted protease inhibitor
- May cause a harmless increase in serum creatinine levels without affecting actual kidney function (glomerular filtration rate)
What Is Tybost and What Is It Used For?
Quick Answer: Tybost (cobicistat) is a pharmacokinetic enhancer used to boost the effectiveness of the HIV protease inhibitors atazanavir and darunavir. It does not fight HIV directly but increases blood levels of these medications, enabling once-daily dosing as part of combination antiretroviral therapy.
Tybost contains the active substance cobicistat, a selective inhibitor of the cytochrome P450 3A (CYP3A) enzyme system. It is indicated for the treatment of human immunodeficiency virus type 1 (HIV-1) infection in adults and adolescents aged 12 years and older who are already receiving antiretroviral therapy. Tybost is specifically designed to be used alongside the protease inhibitors atazanavir or darunavir as part of a complete treatment regimen for HIV.
Unlike ritonavir, which is itself a protease inhibitor used at sub-therapeutic doses as a booster, cobicistat was developed exclusively as a pharmacokinetic enhancer. It has no intrinsic antiviral activity against HIV. Its sole function is to inhibit the CYP3A-mediated metabolism of atazanavir and darunavir, thereby increasing and sustaining their plasma concentrations at therapeutically effective levels throughout the dosing interval.
The concept of pharmacokinetic boosting has been fundamental to modern HIV therapy since the early 2000s. By slowing the enzymatic breakdown of protease inhibitors, booster agents enable lower doses, less frequent dosing, and more predictable drug levels — all of which contribute to improved treatment outcomes and better adherence. Cobicistat provides a targeted alternative to ritonavir boosting, with a more selective enzyme inhibition profile focused specifically on CYP3A.
Tybost is approved for use in patients meeting specific weight requirements: at least 35 kg when used with atazanavir 300 mg, or at least 40 kg when used with darunavir 800 mg. The drug was approved by the European Medicines Agency (EMA) and the U.S. Food and Drug Administration (FDA) and is manufactured by Gilead Sciences. It is available as a single-entity tablet and is also a component of several fixed-dose combination products used in HIV treatment.
What Should You Know Before Taking Tybost?
Quick Answer: Tybost has a wide range of contraindicated medications, must not be used during pregnancy, and requires careful medical supervision. Always inform your healthcare provider about all medications you take, including herbal supplements, before starting Tybost.
Contraindications
Do not take Tybost if you are allergic to cobicistat or any of the other ingredients in the tablet. More critically, Tybost must not be taken together with a long list of medications due to the risk of serious or life-threatening adverse reactions caused by elevated drug levels. These contraindicated medications include:
- Alfuzosin — used for enlarged prostate (risk of severe hypotension)
- Amiodarone, quinidine — antiarrhythmics (risk of cardiac arrhythmias)
- Dabigatran — anticoagulant (risk of bleeding)
- Carbamazepine, phenobarbital, phenytoin — anticonvulsants (will reduce cobicistat levels)
- Rifampicin — anti-tuberculosis agent (will dramatically reduce cobicistat levels)
- Dihydroergotamine, ergotamine, ergometrine — ergot alkaloids (risk of vasospasm)
- St. John’s Wort (Hypericum perforatum) — will reduce cobicistat levels
- Lovastatin, simvastatin — statins (risk of rhabdomyolysis)
- Pimozide, lurasidone — antipsychotics (risk of cardiac and neurological toxicity)
- Sildenafil (for pulmonary arterial hypertension) — risk of visual disturbances and hypotension
- Oral midazolam, triazolam — benzodiazepines (risk of prolonged sedation)
Warnings and Precautions
Your treatment with Tybost must be continuously supervised by a physician experienced in HIV management. Tybost taken with atazanavir or darunavir must not be combined with any other antiviral medication that requires pharmacokinetic boosting, as this could lead to unpredictable drug levels and treatment failure or toxicity.
Inform your healthcare provider if you have or have had kidney disease or abnormal kidney function test results. Cobicistat inhibits the renal tubular secretion of creatinine, which can cause a modest, non-pathological increase in serum creatinine levels. This effect does not represent actual kidney damage, as the glomerular filtration rate (GFR) remains unchanged. However, your physician should be aware of this pharmacological effect to properly interpret your kidney function tests and avoid unnecessary concern or treatment modifications.
Similarly, tell your doctor if you have or have had significant liver disease. Cobicistat is extensively metabolized by the liver, and severe hepatic impairment could alter drug levels and increase the risk of adverse effects. No dose adjustment is necessary for mild to moderate hepatic impairment, but Tybost has not been studied in patients with severe liver disease.
Tybost must not be used concurrently with ritonavir or any other cobicistat-containing product. Using two pharmacokinetic boosters simultaneously provides no additional benefit and increases the risk of drug toxicity and adverse reactions.
Children and Adolescents
Tybost is approved for use in adolescents aged 12 years and older who meet the minimum weight requirements. When used with atazanavir (300 mg), the adolescent must weigh at least 35 kg. When used with darunavir (800 mg), the minimum weight is 40 kg. Tybost has not been studied in children under 12 years of age or in those below the minimum weight thresholds, and its use is therefore not recommended in these populations.
Pregnancy and Breastfeeding
Pregnant women should not take Tybost in combination with atazanavir or darunavir. Clinical data indicate that cobicistat blood levels may decrease significantly during pregnancy, potentially leading to subtherapeutic levels of the boosted protease inhibitor and increased risk of virological failure and perinatal HIV transmission. If you are pregnant, suspect you may be pregnant, or are planning to become pregnant, consult your HIV specialist immediately to discuss alternative treatment options.
Breastfeeding is not recommended during treatment with Tybost. It is unknown whether cobicistat passes into human breast milk. Furthermore, the World Health Organization (WHO) and all major international HIV treatment guidelines recommend that women living with HIV avoid breastfeeding to prevent postnatal transmission of the virus to the infant, regardless of viral load suppression status.
Driving and Operating Machinery
Some patients have reported dizziness when taking Tybost in combination with atazanavir or darunavir. If you experience dizziness or any other neurological symptoms, do not drive or operate machinery until the symptoms resolve. Discuss persistent symptoms with your healthcare provider.
Tybost film-coated tablets contain sunset yellow FCF aluminium lake (E110), which may cause allergic reactions in susceptible individuals. The tablets contain less than 1 mmol (23 mg) sodium per tablet and are therefore essentially sodium-free.
How Does Tybost Interact with Other Drugs?
Quick Answer: Tybost has extensive and clinically significant drug interactions because it potently inhibits CYP3A enzymes. Many common medications are either contraindicated or require dose adjustments when used with Tybost. Always provide your healthcare team with a complete list of all medications, supplements, and herbal products you use.
Cobicistat is a potent, mechanism-based inhibitor of CYP3A. This is the very property that makes it useful as a pharmacokinetic booster, but it also means that Tybost can significantly increase the blood levels of many other medications metabolized by CYP3A. Elevated levels of these drugs can lead to serious, sometimes life-threatening adverse effects. Conversely, strong CYP3A inducers can dramatically reduce cobicistat levels, rendering it ineffective as a booster and potentially leading to HIV treatment failure.
Major Interactions (Contraindicated)
The medications listed in the Contraindications section above must never be taken together with Tybost. These represent absolute contraindications where co-administration could cause severe harm, including cardiac arrhythmias, excessive sedation, severe hypotension, vasospasm, or rhabdomyolysis.
Interactions with Other HIV Medications
Tybost must not be combined with other pharmacokinetic boosters including ritonavir or any other cobicistat-containing product. Additionally, certain antiretroviral medications require special consideration when used alongside a cobicistat-boosted regimen:
- Other protease inhibitors — do not use a second protease inhibitor alongside a cobicistat-boosted regimen
- Efavirenz, etravirine, nevirapine — non-nucleoside reverse transcriptase inhibitors that may alter cobicistat levels; discuss with your physician
- Maraviroc — CCR5 antagonist; dose adjustment may be necessary
Other Important Interactions
Many additional medications require dose adjustments, enhanced monitoring, or careful clinical assessment when used with Tybost. These interactions span a wide range of therapeutic categories:
| Drug Category | Medications | Clinical Consideration |
|---|---|---|
| Antifungals | Ketoconazole, itraconazole, voriconazole, posaconazole, fluconazole | Monitor for toxicity; dose adjustment may be needed |
| Antibiotics | Clarithromycin, rifabutin | Rifabutin dose reduction required; monitor clarithromycin levels |
| Anticancer Agents | Dasatinib, nilotinib, vinblastine, vincristine | Increased risk of chemotherapy toxicity; close monitoring essential |
| Corticosteroids | Betamethasone, budesonide, fluticasone, prednisone, triamcinolone | Risk of Cushing syndrome and adrenal suppression; use alternatives or monitor closely |
| Cardiovascular | Amlodipine, digoxin, diltiazem, metoprolol, verapamil | ECG monitoring; dose reduction may be required |
| Anticoagulants | Apixaban, edoxaban, rivaroxaban, warfarin | Increased bleeding risk; dose adjustment and INR monitoring required |
| Statins | Atorvastatin, fluvastatin, pitavastatin, pravastatin, rosuvastatin | Start with lowest dose; monitor for myopathy symptoms |
| Immunosuppressants | Ciclosporin, sirolimus, tacrolimus | Therapeutic drug monitoring mandatory; significant dose reduction needed |
| Hormonal Contraceptives | Oral and implanted hormonal contraceptives | Efficacy may be reduced; use alternative or additional contraceptive methods |
| Antidiabetics | Metformin | Monitor blood glucose; dose adjustment may be needed |
| PDE5 Inhibitors | Sildenafil (for erectile dysfunction), vardenafil, tadalafil | Significantly reduce dose; start with lowest available dose |
| Sedatives/Anxiolytics | Buspirone, diazepam, zolpidem, clorazepate | Risk of prolonged sedation; dose reduction and monitoring needed |
This table is not exhaustive. The CYP3A inhibitory effect of cobicistat can potentially alter the pharmacokinetics of any medication that is a substrate of CYP3A enzymes. Always consult your HIV specialist or clinical pharmacist before starting, stopping, or changing any medication while taking Tybost. This includes over-the-counter medications, dietary supplements, and herbal products.
What Is the Correct Dosage of Tybost?
Quick Answer: The recommended dose is one 150 mg film-coated tablet taken once daily with food, always in combination with either atazanavir (300 mg) or darunavir (800 mg). The tablet should be swallowed whole — do not chew, crush, or split it.
Adults
Standard Adult Dose
One tablet (150 mg cobicistat) taken once daily by mouth with food. Tybost must be co-administered with either atazanavir 300 mg or darunavir 800 mg at the same time. Food enhances the absorption of cobicistat and ensures consistent drug levels. Always take Tybost as prescribed by your physician — do not alter the dose without medical guidance.
Adolescents (12 to <18 Years)
Adolescent Dose
One tablet (150 mg cobicistat) taken once daily with food. Weight requirements apply:
- With atazanavir 300 mg: the adolescent must weigh at least 35 kg
- With darunavir 800 mg: the adolescent must weigh at least 40 kg
Children Under 12 Years
Tybost is not approved for use in children under 12 years of age or in those below the minimum weight thresholds (35 kg for atazanavir co-administration or 40 kg for darunavir co-administration). Safety and efficacy have not been established in these populations.
Elderly Patients
Limited clinical data are available for patients over 65 years of age. No specific dose adjustment is recommended based on age alone. However, elderly patients often have reduced renal and hepatic function and may be taking multiple medications, increasing the risk of drug interactions. Close monitoring is advised, and a thorough review of concomitant medications is essential before initiating therapy.
| Patient Group | Dose | Requirements |
|---|---|---|
| Adults | 150 mg once daily with food | Co-administer with atazanavir 300 mg or darunavir 800 mg |
| Adolescents (12–17 years) with atazanavir | 150 mg once daily with food | Minimum weight 35 kg |
| Adolescents (12–17 years) with darunavir | 150 mg once daily with food | Minimum weight 40 kg |
| Children <12 years | Not recommended | Safety and efficacy not established |
| Elderly (>65 years) | 150 mg once daily with food | No dose adjustment; review concomitant medications carefully |
Missed Dose
If you miss a dose and notice within 12 hours of your usual dosing time, take the missed dose with food as soon as possible, then continue your regular schedule. If more than 12 hours have passed since the usual dosing time, skip the missed dose entirely and take the next dose at the scheduled time. Never take a double dose to make up for a missed one. Consistent adherence to your HIV treatment regimen is critical for maintaining viral suppression and preventing the development of drug resistance.
Overdose
If you accidentally take more than the prescribed dose of Tybost, contact your physician or go to the nearest emergency department immediately. Bring the medication container so that healthcare providers can easily identify what was taken. There is no specific antidote for cobicistat overdose; treatment is supportive. General measures such as monitoring vital signs, clinical observation, and gastric decontamination (if appropriate and within the timeframe) may be considered.
Stopping Tybost and your protease inhibitor without consulting your physician can lead to rapid viral rebound, development of drug-resistant HIV, and serious consequences for your health and future treatment options. Always ensure you have an adequate supply of your medications, and request refills well in advance.
What Are the Side Effects of Tybost?
Quick Answer: When used with atazanavir, the most common side effects include nausea and jaundice (yellowing of the skin or eyes). Other common effects include headache, dizziness, gastrointestinal problems, rash, and fatigue. Most side effects are mild to moderate and manageable with appropriate medical guidance.
Like all medicines, Tybost can cause side effects, although not everyone experiences them. When treating HIV infection, it can be difficult to determine whether adverse effects are caused by Tybost, the co-administered protease inhibitor (atazanavir or darunavir), other antiretroviral medications in the regimen, or the underlying HIV disease itself. The following side effects have been reported when Tybost is taken with atazanavir:
Very Common
May affect more than 1 in 10 people
- Nausea
- Jaundice (yellowing of the skin and/or whites of the eyes)
Common
May affect up to 1 in 10 people
- High blood sugar (hyperglycaemia)
- Increased appetite, taste disturbances, dry mouth
- Headache, dizziness
- Vomiting, diarrhoea, stomach pain, dyspepsia (indigestion), bloating, flatulence
- Elevated bilirubin levels (hyperbilirubinaemia)
- Skin rash
- Difficulty sleeping, abnormal dreams, drowsiness, fatigue
Uncommon
May affect up to 1 in 100 people
- Blood in the urine (haematuria)
- Protein in the urine (proteinuria)
- Depression
- Itching (pruritus)
- Muscle pain (myalgia), muscle weakness
- Kidney stones (nephrolithiasis)
- Fever
- Sleep disturbances
It is important to understand that the jaundice and elevated bilirubin levels associated with Tybost-boosted atazanavir are primarily caused by atazanavir’s inhibition of the enzyme UGT1A1, which is responsible for bilirubin metabolism. This effect is generally cosmetic (visible yellowing) and not indicative of liver damage, although it can be distressing for patients. If jaundice is bothersome, discuss alternative treatment options with your physician.
Cobicistat inhibits the tubular secretion of creatinine in the kidneys, which may cause a small, clinically insignificant increase in serum creatinine (typically 10–15 micromol/L). This does not represent actual impairment of kidney function. However, if you develop signs of true kidney problems such as significantly reduced urine output, swelling, or unusual fatigue, contact your healthcare provider promptly.
If you experience any side effects, including those not listed here, report them to your healthcare provider. You can also report side effects directly to your national pharmacovigilance authority. Reporting helps to continuously monitor the benefit-risk balance of this medicine.
How Should You Store Tybost?
Quick Answer: Store Tybost at room temperature, out of reach of children. No special storage conditions are required. Keep the desiccant inside the bottle to protect the tablets from moisture.
Store Tybost out of the sight and reach of children. Do not use this medicine after the expiry date stated on the bottle and carton. The expiry date refers to the last day of the stated month. No special storage conditions are required for Tybost tablets. However, the bottle contains a desiccant (silica gel) sachet or canister, which should remain inside the bottle to protect the tablets from moisture — do not swallow the desiccant.
Tybost is supplied in bottles containing 30 film-coated tablets. Outer cartons contain either 1 bottle (30 tablets) or 3 bottles (90 tablets). Not all pack sizes may be marketed in all countries. Do not dispose of medicines via wastewater or household waste. Ask your pharmacist how to properly dispose of medicines that are no longer needed, in order to help protect the environment.
What Does Tybost Contain?
Quick Answer: Each film-coated tablet contains 150 mg of the active substance cobicistat, along with inactive excipients in the tablet core and film coating.
The active substance is cobicistat. Each film-coated tablet contains 150 mg of cobicistat. The inactive ingredients are listed below:
| Component | Ingredients |
|---|---|
| Active Substance | Cobicistat 150 mg |
| Tablet Core | Croscarmellose sodium, magnesium stearate, microcrystalline cellulose (E460), silicon dioxide (E551) |
| Film Coating | Sunset yellow FCF aluminium lake (E110), macrogol 3350 (E1521), polyvinyl alcohol (partially hydrolysed) (E1203), talc (E553), titanium dioxide (E171), yellow iron oxide (E172) |
Tybost tablets are orange, round, biconvex film-coated tablets, debossed with “GSI” on one side and plain on the other. The sunset yellow FCF (E110) dye in the film coating may cause allergic reactions in some individuals. If you have a known allergy to this colouring agent, inform your healthcare provider before starting Tybost.
Frequently Asked Questions About Tybost
Both cobicistat (Tybost) and ritonavir are pharmacokinetic boosters used to enhance the levels of HIV protease inhibitors. The key difference is that cobicistat has no direct antiviral activity against HIV, whereas ritonavir is itself a protease inhibitor used at sub-therapeutic booster doses. Cobicistat was specifically designed as a dedicated booster with more selective CYP3A inhibition. In clinical practice, both boosters achieve similar plasma levels of the co-administered protease inhibitor and comparable virological outcomes. The choice between them often depends on the specific treatment regimen and available fixed-dose combinations.
Cobicistat inhibits the tubular secretion of creatinine in the kidneys by blocking the MATE1 and OCT2 transporters. This causes a modest, reversible increase in serum creatinine (typically 10–15 micromol/L) without actually impairing the glomerular filtration rate (GFR) or true kidney function. This pharmacological effect occurs early after initiation and stabilises over time. It is important that your healthcare provider is aware of this expected effect to avoid misinterpreting routine blood tests as kidney damage.
You should exercise caution with all medications, including over-the-counter products, while taking Tybost. Some common non-prescription medicines, such as St. John’s Wort, can significantly reduce cobicistat levels and compromise HIV treatment. Even seemingly harmless supplements may interact with Tybost’s CYP3A inhibition. Always consult your HIV physician or pharmacist before starting any new medication, supplement, or herbal product.
If you miss a dose and remember within 12 hours of your usual time, take the missed dose with food immediately and continue your regular schedule. If more than 12 hours have passed, skip the missed dose and take your next scheduled dose. Never double up on doses. Missing occasional doses can reduce the effectiveness of your HIV treatment and may contribute to the development of drug resistance. If you frequently miss doses, discuss adherence strategies with your healthcare team.
Yes, cobicistat is available as a component of several fixed-dose combination (FDC) antiretroviral tablets, including Stribild (elvitegravir/cobicistat/emtricitabine/tenofovir disoproxil) and Genvoya (elvitegravir/cobicistat/emtricitabine/tenofovir alafenamide). In these FDC products, cobicistat boosts the integrase inhibitor elvitegravir rather than a protease inhibitor. Tybost as a standalone tablet is used when the prescriber needs to combine cobicistat specifically with atazanavir or darunavir as separate tablets.
No, Tybost does not cure HIV and it does not have any direct antiviral activity. It is a pharmacokinetic booster that enhances the effectiveness of other HIV medications (atazanavir or darunavir). Antiretroviral therapy, including regimens containing Tybost, can suppress the virus to undetectable levels and allow the immune system to recover, but it does not eliminate HIV from the body. Lifelong treatment adherence is necessary to maintain viral suppression.
References
- European Medicines Agency (EMA). Tybost (cobicistat) — Summary of Product Characteristics. Last updated 2023. Available at: EMA — Tybost
- U.S. Food and Drug Administration (FDA). Tybost (cobicistat) Prescribing Information. Gilead Sciences, Inc. Revised 2024.
- European AIDS Clinical Society (EACS). Guidelines Version 12.0, October 2024. Available at: EACS Guidelines
- Panel on Antiretroviral Guidelines for Adults and Adolescents. Guidelines for the Use of Antiretroviral Agents in Adults and Adolescents with HIV. Department of Health and Human Services (DHHS). Updated 2024.
- World Health Organization (WHO). Consolidated Guidelines on HIV Prevention, Testing, Treatment, Service Delivery and Monitoring: Recommendations for a Public Health Approach. WHO, 2024.
- Lepist EI, Phan TK, Roy A, et al. Cobicistat boosts the intestinal absorption of transport substrates, including HIV protease inhibitors and GS-7340, in vitro. Antimicrobial Agents and Chemotherapy. 2012;56(10):5409–5413.
- Gallant JE, Koenig E, Andrade-Villanueva JF, et al. Cobicistat versus ritonavir as pharmacoenhancers of atazanavir plus emtricitabine/tenofovir disoproxil fumarate in treatment-naive HIV type 1-infected patients: week 144 results. Journal of Infectious Diseases. 2015;211(8):1243–1250.
- British National Formulary (BNF). Cobicistat. NICE Evidence Services. Last reviewed 2025.
Editorial Team
Medical Content
iMedic Medical Editorial Team — Specialists in Infectious Disease and Clinical Pharmacology
Medical Review
iMedic Medical Review Board — Independent panel of medical experts reviewing all content
Evidence Framework
GRADE methodology — Level 1A evidence based on systematic reviews and randomized controlled trials
Guidelines
EMA SmPC, FDA Label, EACS Guidelines 2024, DHHS Guidelines, WHO HIV Guidelines
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