Tecovirimat SIGA (Tecovirimat)

What Is Tecovirimat SIGA and What Is It Used For?

Tecovirimat SIGA is an oral antiviral medicine used to treat orthopoxvirus infections in adults and children weighing at least 13 kg. It is approved for smallpox, mpox (monkeypox), cowpox, and complications of smallpox vaccination. Tecovirimat specifically blocks the VP37 protein on poxvirus envelopes and is the first antiviral developed primarily for this family of viruses.

Tecovirimat represents a focused therapeutic response to a distinctive family of pathogens, the orthopoxviruses. This genus includes variola virus, the agent of smallpox, which was eradicated from circulating human populations in 1980 but remains a biosecurity concern. It also includes monkeypox virus, which caused a multi-country outbreak beginning in 2022 and is now an ongoing global health issue, as well as cowpox, vaccinia (used in smallpox vaccines), camelpox, and several less common zoonotic poxviruses. Because orthopoxviruses share a conserved gene encoding the VP37 envelope wrapping protein, a single small-molecule inhibitor can potentially cover them all.

The active substance, tecovirimat, was developed under the codename ST-246 through a United States medical countermeasures program aimed at preparedness against bioterrorism involving smallpox. The brand name TPOXX is used in the United States, while Tecovirimat SIGA is the name under which the European Medicines Agency granted marketing authorization. Both products contain the same active ingredient and share the same 200 mg hard capsule formulation.

Regulatory approval pathways for tecovirimat have been unusual. The U.S. Food and Drug Administration (FDA) approved TPOXX in 2018 for the treatment of smallpox under the Animal Rule, which allows efficacy to be established from studies in well-characterized animal models when human trials are impossible for ethical reasons. The European Medicines Agency (EMA) granted conditional marketing authorization in 2022 for tecovirimat in smallpox, mpox, cowpox, and vaccinia complications, based on animal efficacy data, human pharmacokinetics, and safety data. Both agencies classified it as an orphan medicinal product.

Approved Indications

Tecovirimat SIGA is authorized for the treatment of the following orthopoxvirus diseases in adults and children weighing 13 kg or more:

• Smallpox (variola major and minor) — a serious systemic viral illness with a characteristic vesicular rash; no naturally occurring cases have been reported since 1977, but stockpiling antivirals is considered prudent preparedness
• Mpox (monkeypox) — a zoonotic disease endemic in parts of Central and West Africa and, since 2022, causing sustained human-to-human transmission globally, particularly among sexual networks and close household contacts
• Cowpox — a mainly European zoonotic infection, typically transmitted from small mammals such as rats or domestic cats, causing localized ulcerative skin lesions
• Complications following immunization with vaccinia-based smallpox vaccines — these include progressive vaccinia, eczema vaccinatum, and generalized vaccinia, which can occur in individuals with compromised skin barrier or immune function

Tecovirimat does not treat chickenpox or shingles (which are caused by the varicella-zoster virus), herpes simplex infections, molluscum contagiosum, or unrelated viral respiratory illnesses such as influenza or COVID-19.

How Tecovirimat Works

All orthopoxviruses replicate in the cytoplasm of infected cells and produce two types of infectious virus particles. Intracellular mature virions (IMV) are the most abundant form and spread mainly when infected cells burst open. Extracellular enveloped virions (EEV) are created when IMV are wrapped in an additional lipid membrane containing viral proteins, including VP37 (encoded by the F13L gene in vaccinia and conserved homologues in other orthopoxviruses). EEV are responsible for long-range spread of virus between cells and throughout the body.

Tecovirimat binds directly to VP37 and disrupts its interaction with cellular Rab9 GTPase and TIP47, proteins that normally help assemble the enveloped form of the virus. With VP37 inhibited, infected cells can still produce IMV but cannot efficiently wrap them, so the enveloped virions needed for systemic spread are not made. In animal models of lethal orthopoxvirus disease, this blockade translates into reduced viral titers, less severe disease, and improved survival even when dosing begins after symptoms have started.

Because VP37 has no close human counterpart, tecovirimat has a high selectivity index: it inhibits poxvirus replication at concentrations hundreds of times lower than those that affect human cells. This selectivity explains the relatively modest side effect profile reported in clinical studies and healthy volunteer trials.

Clinical Evidence in Humans

Because smallpox is no longer circulating, efficacy in humans against variola cannot be studied. For smallpox, approval rests on animal models in non-human primates and rabbits, supported by robust human pharmacokinetic and safety data. Exposures associated with protection in animals have been bridged to humans using plasma concentrations.

For mpox, two pivotal randomized placebo-controlled trials have been completed: the PALM007 trial in the Democratic Republic of the Congo, which enrolled adults and children with clade I mpox, and the STOMP trial in immunocompetent adults with clade II mpox in the United States and several other countries. Neither study demonstrated a statistically significant reduction in time to lesion resolution with tecovirimat compared to placebo. These findings tempered the enthusiasm that had followed early observational reports but did not overturn approvals, because animal efficacy data remain compelling and clinicians continue to use tecovirimat in severe mpox, in immunocompromised patients, and under expanded access protocols.

What Should You Know Before Taking Tecovirimat SIGA?

Before starting tecovirimat, tell your doctor about all medicines you take, your medical history (especially kidney or liver disease, diabetes, and any bleeding disorders), whether you are pregnant, planning pregnancy, or breastfeeding, and any allergies. A full medication review is essential because tecovirimat interacts with enzymes that process many common drugs.

Tecovirimat is generally well tolerated in the adults and older children who were studied in clinical trials, but responsible prescribing requires a careful pre-treatment assessment. The objective is to confirm the diagnosis, rule out contraindications, identify drug interactions, and plan monitoring appropriate to the patient's individual situation.

Contraindications

You must not take Tecovirimat SIGA if:

• You are hypersensitive (allergic) to tecovirimat or to any of the excipients listed in the composition section
• You weigh less than 13 kg; there are no approved dosing recommendations for infants and very small children below this weight

There are currently no other absolute contraindications listed in the European Summary of Product Characteristics, but the following clinical situations require additional caution.

Warnings and Precautions

Discuss the following with your healthcare professional before starting tecovirimat:

• Hepatic impairment: Tecovirimat has not been studied in patients with moderate or severe hepatic impairment (Child-Pugh class B and C). Use with caution; close monitoring may be needed
• Severe renal impairment: Data are limited in patients with severe renal impairment (eGFR below 30 mL/min/1.73 m²) and in those on dialysis. Dose adjustment is not usually required, but individual assessment is advisable
• Diabetes or glucose-lowering medicines: Because tecovirimat inhibits CYP2C8, it increases exposure to the oral antidiabetic drug repaglinide and can cause hypoglycemia. Blood glucose should be monitored closely if repaglinide cannot be substituted
• History of seizure disorders: Seizures were observed in some early-phase studies at higher doses and in a small proportion of patients in expanded access programs, although a causal relationship has not been established. Tell your doctor if you have a history of epilepsy or unexplained convulsions
• Depression or psychiatric conditions: Depression and suicidal ideation have been rarely reported during post-marketing surveillance. Report any new or worsening mood symptoms promptly
• Hormonal contraception: Tecovirimat is a weak inducer of CYP3A4 and may reduce the effectiveness of some hormonal contraceptives. Use an additional non-hormonal method (such as condoms) during treatment and for at least 14 days after the last dose
• HIV infection: Tecovirimat can be used in people living with HIV, but interactions with antiretroviral therapy (especially regimens metabolized via CYP3A4 or CYP2C9/2C19) should be reviewed by an HIV specialist
• Concurrent smallpox vaccination: If vaccinia vaccine is given while on tecovirimat, the antiviral may theoretically reduce the immunogenicity of the live vaccine. Consult a vaccination specialist on timing

Pregnancy and Breastfeeding

Clinical experience with tecovirimat in pregnancy is limited. Animal reproductive studies have not shown teratogenic effects, but human data are insufficient to make definitive recommendations. Pregnancy itself does not protect against severe orthopoxvirus disease, and untreated smallpox or severe mpox in pregnancy is associated with fetal loss, preterm birth, and congenital mpox infection.

The decision to use tecovirimat in pregnancy should be individualized, balancing the potential risks of fetal exposure against the expected benefits of treating a serious viral infection. In severe maternal disease or high-risk exposure, treatment is typically offered. Pregnant individuals treated with tecovirimat are encouraged to register with pregnancy exposure registries where available.

It is not known whether tecovirimat is excreted in human breast milk. Animal lactation studies suggest minimal drug transfer. For most patients, shared decision-making about temporary interruption of breastfeeding versus continuing during treatment is appropriate, and direct contact between breastfeeding infants and any active skin lesions should always be avoided.

Fertility

No adverse effects on fertility have been observed in animal studies. Human fertility data are not available. For patients trying to conceive, tecovirimat is not considered a major concern, but the underlying infection and any concomitant treatments should be reviewed with a fertility specialist.

Driving and Operating Machinery

Tecovirimat itself is unlikely to impair the ability to drive or use machines. However, some patients report dizziness, headache, or fatigue. If you experience these symptoms, avoid driving or operating heavy equipment until they resolve. The underlying illness being treated may itself make it inadvisable to drive.

Children and Adolescents

Tecovirimat SIGA is approved for children weighing 13 kg or more, with weight-based dosing (see the dosage section below). The hard capsule may be opened and the contents mixed with soft food such as apple puree or yogurt for children who cannot swallow capsules whole; an oral liquid formulation has also been developed in some markets to improve paediatric dosing accuracy. Safety and efficacy have not been established in infants weighing less than 13 kg.

Elderly Patients

There are no specific dose adjustments for older adults. However, elderly patients are more likely to have reduced renal or hepatic function, comorbidities, and polypharmacy. A careful medication review, especially for CYP3A4, CYP2C8, and CYP2C19 substrates, is recommended before starting tecovirimat in this population.

How Does Tecovirimat SIGA Interact with Other Drugs?

Tecovirimat is a weak inducer of CYP3A4 and an inhibitor of CYP2C8 and CYP2C19. This can increase exposure to drugs such as repaglinide (risk of hypoglycemia) and decrease exposure to drugs such as midazolam and some hormonal contraceptives. Always review all medicines with your doctor before starting treatment.

Unlike many antiviral drugs, tecovirimat does not use CYP enzymes as its main route of elimination; it is primarily glucuronidated. However, in vitro and clinical interaction studies show that it modulates several drug-metabolizing enzymes at therapeutic doses. The clinical relevance of these interactions varies from negligible to potentially serious, and the net effect depends on the other drug's narrow therapeutic index and the patient's individual situation.

Major Interactions

Minor Interactions and Supplements

No clinically important interactions have been reported between tecovirimat and commonly used supplements such as vitamin D, calcium, or iron. Grapefruit juice, despite being a potent CYP3A4 inhibitor relevant to many drugs, is not considered to meaningfully change tecovirimat exposure. However, because tecovirimat needs to be taken with a fatty meal, heavy use of grapefruit juice alone is not a substitute for food.

Alcohol has not been studied directly with tecovirimat but is generally best avoided or limited during any acute infection, both for symptomatic reasons and because it can add to gastrointestinal side effects such as nausea.

What to Do If You Take Interacting Medicines

If you already take one of the medicines listed above, do not stop your regular treatment without medical advice. In most cases, tecovirimat can be started alongside existing therapy with appropriate monitoring. Your prescriber may:

• Temporarily switch a narrow-therapeutic-index drug (for example, swap repaglinide for another antidiabetic)
• Check blood levels of the affected drug (for example, INR for warfarin)
• Adjust the dose of the affected drug temporarily during and shortly after the 14-day course
• Advise additional non-hormonal contraception for the treatment period and 14 days afterwards

What Is the Correct Dosage of Tecovirimat SIGA?

The standard adult dose for patients weighing 40 kg or more is 600 mg (three 200 mg capsules) twice daily for 14 days, taken within 30 minutes after a full meal with moderate or high fat content. Pediatric doses are weight-based. Missing doses or taking the medication without fatty food can significantly reduce its effectiveness.

Dosing of tecovirimat is straightforward compared with many antivirals: the total duration is fixed at 14 days regardless of when treatment is started during the illness, and dosing does not need to be adjusted for mild or moderate renal or hepatic impairment. What does matter, and what patients frequently get wrong, is the relationship with food. Tecovirimat absorption is poor without a fatty meal, so the clinical success of the course depends not only on taking the right dose but also on how each dose is taken.

Adults (40 kg and Above)

Children 25 kg to Less Than 40 kg

Children 13 kg to Less Than 25 kg

Children Less Than 13 kg

Elderly Patients

Renal and Hepatic Impairment

How to Take the Capsules

• Swallow the capsules whole with water, preferably 10 to 30 minutes after finishing a fatty meal
• If you cannot swallow capsules, open each capsule and mix the entire contents with 30 mL of apple puree, yogurt, or chocolate milk; take the whole mixture within 30 minutes
• Do not chew the capsule contents; they have a bitter taste
• Do not crush or split the capsules before swallowing unless you are using the open-and-sprinkle method
• Space morning and evening doses as close to 12 hours apart as possible

Missed Dose

If you miss a dose and remember within 6 hours of the scheduled time, take the missed dose with a suitable fatty meal as soon as possible, then continue with your usual schedule. If more than 6 hours have passed, skip the missed dose and take the next dose at the usual time. Do not double the dose to make up for a missed one. Keep a record of any missed doses so you can discuss them with your healthcare provider if you are concerned.

Overdose

There is no specific antidote for tecovirimat overdose. In clinical studies, single doses up to 2,000 mg and multiple doses up to 1,000 mg per day for 21 days have been tolerated without serious toxicity. If an overdose occurs, contact your local poison control center or emergency department. Treatment is supportive and typically involves observation, maintenance of hydration, and management of any symptoms such as nausea or headache.

Duration of Treatment

The full 14-day course should be completed even if symptoms improve earlier. Stopping treatment prematurely may allow viral replication to rebound. In rare cases of treatment failure, extended courses, combination therapy with brincidofovir or vaccinia immune globulin, or consideration of resistance testing may be warranted; these decisions are made by specialists.

What Are the Side Effects of Tecovirimat SIGA?

The most common side effects are headache and nausea, each affecting more than 1 in 10 patients. Other common reactions include abdominal pain, vomiting, diarrhea, dizziness, and rash. Serious hypersensitivity reactions, depression, and seizures have been reported rarely. Most side effects are mild to moderate and resolve after the 14-day course ends.

Tecovirimat has a favorable safety profile in the healthy volunteer and patient populations studied to date. Side effects are generally mild, short-lived, and manageable with supportive care. Because the drug is taken for only 14 days, cumulative toxicity is not a major concern. Nevertheless, recognizing potential adverse reactions helps patients distinguish expected drug effects from signs of worsening infection.

Frequency Categories

Adverse reactions are classified according to how often they were reported in clinical trials and post-marketing surveillance, following international conventions used by the European Medicines Agency:

Interpreting Side Effects During Orthopoxvirus Infection

A particular challenge when treating mpox or smallpox is that many symptoms of the underlying illness overlap with potential drug side effects. Headache, fatigue, nausea, and abdominal discomfort are common in viral infections themselves. Clinicians rely on timing, severity, and pattern to distinguish drug-related effects: symptoms that begin or worsen sharply within one or two days of starting tecovirimat, or that resolve quickly after stopping the drug, are more suggestive of a medication reaction.

Rashes deserve particular attention. Orthopoxvirus infections cause their own characteristic skin eruptions, and new lesions continuing to appear on days three to seven of illness are expected. A generalized urticarial or morbilliform rash, especially with itching, facial swelling, or respiratory symptoms, is more suggestive of drug hypersensitivity and should be reported immediately.

Laboratory Monitoring

Routine blood tests are not mandatory for short courses of tecovirimat in healthy adults. However, in hospitalized patients, patients with pre-existing liver disease, or those with comorbidities, prescribers may elect to check liver enzymes (ALT, AST), renal function (creatinine, eGFR), and complete blood counts at baseline and once during the treatment course.

Reporting Suspected Side Effects

If you suspect any side effect from tecovirimat, discuss it with your doctor, pharmacist, or nurse. Reporting side effects through national pharmacovigilance systems (for example, the EudraVigilance system in Europe, the MHRA Yellow Card Scheme in the UK, or the FDA MedWatch program in the United States) helps health authorities understand the long-term safety profile of newer antivirals and protect future patients.

How Should You Store Tecovirimat SIGA?

Store Tecovirimat SIGA capsules in the original blister pack below 25°C, protected from light and moisture. Keep the medicine out of the sight and reach of children. Do not use after the expiration date printed on the carton and blister. Return any unused capsules to a pharmacy or an authorized medicine disposal program.

Proper storage preserves the potency of tecovirimat for the duration of its approved shelf life, which is an important consideration for national strategic stockpiles as well as for individual patients who receive the drug through expanded access programs or hospital dispensing.

Storage Conditions

• Temperature: Store below 25°C (77°F). Brief excursions up to 30°C during transport are tolerated. Do not freeze
• Humidity: Keep capsules in the original aluminium blister pack until immediately before use to protect from moisture
• Light: Store in the original carton to protect from light
• Child safety: Keep the medicine out of the sight and reach of children. Accidental ingestion can occur if capsules are left on counters or bedside tables

Expiration and Stability

The expiration date is printed on the carton and the blister in the format MM/YYYY. The medicine should not be used after the last day of the month indicated. The shelf life of unopened, properly stored capsules is typically five years from the date of manufacture, which enables long-term storage in national stockpiles. Once a blister is opened, the capsule inside should be taken promptly as part of the scheduled dose; tecovirimat is not sensitive to light or air for the few minutes required to swallow a dose.

Travel with Tecovirimat

If you need to travel during your 14-day course, keep the medicine in your carry-on luggage in its original packaging, together with a copy of the prescription or clinical summary. Avoid placing capsules in checked luggage where they may be exposed to extreme temperatures. Temperature-sensitive travel bags are not generally necessary for short trips.

Disposal

Do not throw away medicines via wastewater or household waste. Unused or expired capsules should be returned to a pharmacy or handed to a local authorized medicine take-back program. These measures help protect the environment and prevent accidental exposure. Do not flush capsules down the toilet or pour them into sinks.

What Does Tecovirimat SIGA Contain?

Each hard capsule contains 200 mg of tecovirimat as the active ingredient, formulated with microcrystalline cellulose, croscarmellose sodium, hypromellose, lactose monohydrate, magnesium stearate, and colloidal silicon dioxide. The capsule shell is made of gelatin, titanium dioxide, and shellac-based printing ink.

Understanding what is inside each capsule is useful both for patients with specific allergies and for people who follow dietary or religious restrictions. The formulation of Tecovirimat SIGA is deliberately simple: the active molecule is not very soluble in water and therefore requires careful pharmaceutical engineering to ensure reproducible absorption, especially when taken with food.

Active Ingredient

Each capsule contains 200 mg of tecovirimat (equivalent to 210 mg of tecovirimat monohydrate). Tecovirimat is a small-molecule compound of the adamantyl class and has the chemical name 4-(trifluoromethyl)-N-((3aR,4R,4aR,5aS,6S,6aS)-3-oxo-2-octahydro-2,5,6-trimethano-cyclopropa[a]pentalen-3a(3H)-yl)benzamide. Its full IUPAC name and molecular structure are detailed in the Summary of Product Characteristics.

Excipients (Inactive Ingredients)

The capsule contents contain the following inactive ingredients:

• Microcrystalline cellulose — a filler and binder derived from plant cellulose
• Lactose monohydrate — a diluent; patients with rare hereditary problems of galactose intolerance, total lactase deficiency, or glucose-galactose malabsorption should not take this medicine
• Croscarmellose sodium — a disintegrant that helps the capsule break apart quickly after swallowing
• Hypromellose (HPMC) — a binder that improves tablet structure and dissolution
• Magnesium stearate — a lubricant that prevents sticking during manufacture
• Colloidal anhydrous silica — a glidant that improves powder flow

Capsule Shell

The capsule shell contains:

• Gelatin — derived from bovine or porcine sources; patients following vegetarian or vegan diets, or those with religious dietary laws restricting certain animal products, should discuss alternatives with their doctor
• Titanium dioxide (E171) — a white opacifier
• Printing ink — containing shellac, black iron oxide (E172), propylene glycol, and potassium hydroxide

Allergen and Dietary Considerations

• Lactose: each capsule contains a small amount of lactose; not suitable for patients with severe lactose intolerance or hereditary galactose disorders
• Gluten-free: the formulation does not contain gluten
• Gelatin source: confirm the source with the prescriber or pharmacist if dietary restrictions apply
• Sodium content: negligible amount from croscarmellose sodium; suitable for sodium-restricted diets

Appearance and Packaging

Tecovirimat SIGA capsules are size 0 hard gelatin capsules with an opaque orange cap and opaque off-white body, printed with “SIGA” on the cap and “ST-246” on the body in black ink. Each capsule contains a white to off-white powder. Capsules are packaged in PVC/aluminium blister strips, with each carton typically containing sufficient capsules for the full 14-day course.