Doxycycline: Uses, Dosage & Side Effects

What Is Doxycycline and What Is It Used For?

Doxycycline is a tetracycline antibiotic that works by preventing bacteria from producing the proteins they need to grow and multiply. It is used to treat infections of the lungs, urinary tract, skin and soft tissues, as well as Lyme disease, sexually transmitted infections and malaria prophylaxis.

Doxycycline belongs to the tetracycline family of antibiotics. Unlike bactericidal antibiotics that kill bacteria directly, doxycycline is bacteriostatic, meaning it stops bacteria from growing and reproducing. It does this by binding to the 30S ribosomal subunit inside bacterial cells, which blocks the attachment of transfer RNA (tRNA) to the messenger RNA-ribosome complex. Without this critical step, bacteria cannot synthesise the proteins they need to survive and replicate.

One of the major clinical advantages of doxycycline is its remarkably broad spectrum of activity. It is effective against a wide range of Gram-positive bacteria (such as Streptococcus pneumoniae and Staphylococcus aureus), Gram-negative bacteria (such as Haemophilus influenzae and Neisseria gonorrhoeae), atypical organisms (including Chlamydia trachomatis, Mycoplasma pneumoniae and Borrelia burgdorferi), and even certain protozoa such as Plasmodium species that cause malaria.

Doxycycline is well absorbed from the gastrointestinal tract, even when taken with food, which is a significant advantage over older tetracyclines. It has a long half-life of approximately 18–22 hours, allowing for convenient once- or twice-daily dosing. The drug is widely distributed throughout the body, achieving therapeutic concentrations in the lungs, sinuses, skin, urogenital tract and other tissues.

Common indications for doxycycline

Doxycycline is one of the most versatile antibiotics available and is prescribed for a diverse range of conditions. The most common therapeutic uses include:

• Lyme disease (Borrelia infection): First-line treatment for early Lyme disease following a tick bite, typically prescribed for 10–21 days depending on the stage of infection
• Chlamydia and other sexually transmitted infections: Effective against Chlamydia trachomatis, Ureaplasma urealyticum, and as an alternative treatment for syphilis and gonorrhoea
• Community-acquired pneumonia: Used when atypical pathogens are suspected or in patients allergic to penicillin
• Acute bacterial sinusitis: Second-line treatment for patients who are allergic to penicillins or when first-line therapy has failed
• Acute exacerbation of chronic bronchitis: When bacterial infection is contributing to worsening of underlying chronic obstructive pulmonary disease
• Acne vulgaris: Moderate-to-severe inflammatory acne, usually prescribed for 6–12 weeks
• Rosacea: At sub-antimicrobial doses (40 mg modified-release), doxycycline reduces the inflammatory lesions of rosacea
• Malaria prophylaxis: Recommended for travellers visiting areas with chloroquine-resistant Plasmodium falciparum malaria
• Urinary tract infections: When susceptibility testing confirms doxycycline sensitivity
• Periodontal disease: As an adjunct to scaling and root planing at sub-antimicrobial doses

What Should You Know Before Taking Doxycycline?

Before taking doxycycline, tell your doctor if you are pregnant, breastfeeding, have liver disease, myasthenia gravis, or are allergic to tetracycline antibiotics. Doxycycline is contraindicated in pregnancy, during breastfeeding, and generally in children under 8 years of age.

Doxycycline is a highly effective antibiotic, but it is not suitable for everyone. Before starting treatment, your prescribing doctor needs to be aware of your complete medical history, current medications and any allergies. Certain medical conditions and life stages make doxycycline unsafe or require careful dose adjustment and monitoring throughout the course of treatment.

Contraindications

You must not take doxycycline if any of the following apply to you:

• Allergy to doxycycline or any tetracycline antibiotic: If you have previously experienced an allergic reaction to doxycycline, tetracycline, minocycline, or any other tetracycline-class antibiotic, you must not take this medication. Symptoms of an allergic reaction include rash, itching, swelling of the face or throat, and difficulty breathing.
• Pregnancy: Doxycycline crosses the placenta and can cause permanent tooth discolouration (yellow-grey-brown staining), enamel hypoplasia and adverse effects on skeletal development in the foetus. These effects are most significant during the second and third trimesters, but doxycycline should be avoided throughout pregnancy.
• Breastfeeding: Doxycycline passes into breast milk and may affect bone and tooth development in the nursing infant. Alternative antibiotics that are safe during breastfeeding should be used instead.
• Children under 8 years of age: Tetracycline antibiotics bind permanently to developing teeth and bones, causing irreversible tooth discolouration and potentially affecting skeletal growth. In rare, serious or life-threatening situations where no alternative exists, the prescribing physician may determine that the benefits outweigh these risks.

Warnings and Precautions

Talk to your doctor before taking doxycycline if you have any of the following conditions, as your treatment may need to be modified or closely monitored:

• Myasthenia gravis: Doxycycline may worsen muscle weakness in patients with this neuromuscular condition. If you have myasthenia gravis, your doctor will carefully weigh the risks and benefits before prescribing.
• Liver disease or impaired liver function: Doxycycline is partially metabolised by the liver. In patients with significant hepatic impairment, the drug may accumulate to toxic levels. Your doctor may need to adjust the dose or choose an alternative antibiotic.
• Oesophageal conditions: Doxycycline can cause severe oesophageal ulceration and irritation if the capsule or tablet lodges in the oesophagus. Always take doxycycline with a full glass of water (at least 200 ml) and remain in an upright position for at least 30 minutes after swallowing the dose.

Pregnancy and Breastfeeding

Doxycycline is strictly contraindicated during pregnancy and breastfeeding. The evidence for these restrictions is well established and supported by all major regulatory agencies worldwide, including the EMA, FDA, WHO and MHRA.

During pregnancy, doxycycline crosses the placental barrier and accumulates in developing foetal teeth and bones. This can cause permanent yellow-grey-brown discolouration of teeth and enamel hypoplasia (defective tooth enamel) in the child. Additionally, tetracyclines can be deposited in growing bone, which may temporarily inhibit skeletal growth. These effects are particularly concerning during the second and third trimesters when tooth and bone mineralisation is actively occurring.

During breastfeeding, doxycycline is excreted in breast milk, though concentrations are relatively low. The American Academy of Pediatrics has noted that short courses may be acceptable in certain circumstances, but most guidelines recommend avoiding doxycycline during lactation due to the theoretical risk to the infant's developing teeth and bones. Your doctor will prescribe a safer alternative such as amoxicillin, azithromycin or another antibiotic compatible with breastfeeding.

If you discover you are pregnant while taking doxycycline, stop the medication immediately and contact your doctor. A single dose or very short exposure in early pregnancy is unlikely to cause harm, but continued use should be avoided.

Driving and operating machinery

Doxycycline is not known to impair the ability to drive or operate machinery. However, as with any medication, you should be aware of how it affects you personally before engaging in activities requiring alertness. If you experience dizziness, blurred vision or other side effects that could impair concentration, refrain from driving until these effects resolve.

How Does Doxycycline Interact with Other Drugs?

Doxycycline has clinically significant interactions with antacids, iron supplements, calcium, warfarin, isotretinoin, rifampicin and anti-epileptic drugs. Antacids and minerals reduce absorption, while certain enzyme-inducing drugs can reduce doxycycline's effectiveness.

Drug interactions are an important consideration with doxycycline because several commonly used medications can either reduce its effectiveness or increase the risk of adverse effects. Before starting doxycycline, you should provide your doctor and pharmacist with a complete list of all medications, supplements and herbal products you are currently taking. This includes prescription drugs, over-the-counter medicines, vitamins and dietary supplements.

Major Interactions – Avoid Concurrent Use

The following medications should be avoided while taking doxycycline, or require careful management if co-administration is necessary:

Moderate Interactions – Use with Caution

The following drugs may interact with doxycycline and require dose adjustment, monitoring or careful timing of administration:

Doxycycline and alcohol

The effectiveness of doxycycline may be reduced when taken with alcohol. While the interaction is not as severe as with some other antibiotics, alcohol can increase the hepatic metabolism of doxycycline, particularly in heavy drinkers, potentially leading to subtherapeutic blood levels. Alcohol can also exacerbate gastrointestinal side effects such as nausea, vomiting and stomach upset. For the best treatment outcome, it is advisable to minimise or avoid alcohol consumption during the course of antibiotic therapy.

What Is the Correct Dosage of Doxycycline?

The standard adult dose of doxycycline is 200 mg on the first day (as a loading dose), followed by 100 mg once daily. Dosage varies by indication: Lyme disease typically requires 100 mg twice daily for 14–21 days, while chlamydia is treated with 100 mg twice daily for 7 days.

The dose of doxycycline is determined by your doctor based on the type and severity of your infection, your age, body weight and kidney/liver function. It is crucial to follow your prescribed dosage exactly and to complete the full course of treatment, even if you start feeling better before the course is finished. Stopping antibiotics early increases the risk of antibiotic resistance and may allow the infection to return.

Adults and Adolescents (Over 12 Years)

The standard dosing regimen for most infections in adults is a loading dose of 200 mg on the first day (given as a single dose or as 100 mg every 12 hours), followed by a maintenance dose of 100 mg once daily. For more severe infections, the higher dose of 200 mg daily may be continued throughout the treatment course.

Children (8 to 12 Years)

Doxycycline should only be used in children aged 8 to 12 years when other antibiotics are not available, not appropriate, or unlikely to be effective. Under these circumstances, the dosage is based on body weight:

Elderly Patients

No specific dose adjustment is required for elderly patients with normal liver and kidney function. However, elderly individuals may be more susceptible to oesophageal irritation, so extra care should be taken to swallow the medication with plenty of water and to remain upright afterwards. Elderly patients on anticoagulant therapy (such as warfarin) should have their INR monitored more frequently when starting or stopping doxycycline.

Missed Dose

If you miss a dose of doxycycline, take it as soon as you remember. If it is nearly time for the next scheduled dose, skip the missed dose and take the next dose at the regular time. Do not take a double dose to compensate for the one you missed. Consistently missing doses may reduce the effectiveness of the antibiotic and contribute to the development of resistant bacteria.

Overdose

Symptoms of doxycycline overdose may include severe nausea, vomiting and diarrhoea. If you suspect an overdose, contact your local poison control centre or seek emergency medical attention immediately. Treatment is primarily supportive, as doxycycline is not efficiently removed by haemodialysis or peritoneal dialysis.

What Are the Side Effects of Doxycycline?

The most common side effects of doxycycline are nausea, vomiting and diarrhoea (affecting up to 1 in 10 patients). Less common effects include photosensitivity skin reactions and rash. Rare but serious side effects include severe allergic reactions, oesophageal ulceration and increased intracranial pressure.

Like all medicines, doxycycline can cause side effects, although not everyone experiences them. Most side effects are mild and resolve on their own once the course of treatment is completed. However, some side effects require immediate medical attention. If you experience any symptoms that concern you, contact your doctor or pharmacist promptly.

If you notice prolonged or troublesome diarrhoea during or after your course of doxycycline, contact your doctor. This may indicate antibiotic-associated colitis (including Clostridioides difficile infection), which requires prompt medical evaluation and treatment.

Fungal overgrowth (candidiasis) of the mouth or vagina may occasionally occur because doxycycline disrupts the normal bacterial flora, allowing opportunistic fungi to proliferate. If thrush develops and persists for several days or is severe, contact your doctor for antifungal treatment.

How Should You Store Doxycycline?

Store doxycycline capsules and tablets at room temperature (below 25°C), away from direct sunlight and moisture. Keep out of reach of children. Do not use after the expiry date. The concentrate for intravenous infusion must be stored in a refrigerator (2–8°C) and protected from light.

Proper storage is essential to ensure that doxycycline remains effective and safe throughout its shelf life. Degraded tetracycline antibiotics can become toxic, potentially causing a condition known as Fanconi syndrome (renal tubular damage). While this risk is primarily associated with expired tetracycline rather than doxycycline specifically, it underscores the importance of not using any antibiotic past its expiry date.

• Capsules and tablets: Store at room temperature (15–25°C) in the original packaging to protect from moisture and light. Do not store in the bathroom, as humidity can degrade the medication.
• Oral suspension: Once reconstituted, store in a refrigerator and use within 14 days unless otherwise directed by the manufacturer.
• Concentrate for IV infusion: Store in a refrigerator (2–8°C) in the original carton to protect from light. After dilution, the infusion solution should be used within 6 hours and protected from direct sunlight.
• General rules: Keep all medications out of sight and reach of children. Do not use doxycycline after the expiry date printed on the packaging. Do not dispose of medications via household waste or down the drain – return unused medicines to your pharmacy for safe disposal.

What Does Doxycycline Contain?

The active ingredient is doxycycline, available as doxycycline hyclate or doxycycline monohydrate. Inactive ingredients vary by formulation and manufacturer but typically include lactose, magnesium stearate, microcrystalline cellulose and various capsule shell components.

Doxycycline is available in multiple formulations from different manufacturers. The active substance is doxycycline itself, though it is formulated as different salt forms depending on the product:

• Doxycycline hyclate (hydrochloride hemiethanolate hemihydrate): The most common salt form used in oral capsules, tablets and the intravenous concentrate. Doxycycline hyclate dissolves readily in water and is rapidly absorbed.
• Doxycycline monohydrate: Used in some oral formulations, particularly those designed for improved gastrointestinal tolerability. It may cause slightly less oesophageal irritation compared to the hyclate form.

For the intravenous formulation (concentrate for infusion at 20 mg/ml), the additional inactive ingredients include ethanolamine, magnesium chloride hexahydrate, povidone, acetylcysteine and water for injections. The concentrate appears as a clear, yellowish liquid and is supplied in glass ampoules containing 5 ml (equivalent to 100 mg doxycycline).

Common inactive ingredients in oral capsule and tablet formulations may include:

• Microcrystalline cellulose
• Lactose monohydrate
• Magnesium stearate
• Sodium lauryl sulfate
• Povidone
• Gelatin and titanium dioxide (capsule shell)

If you have known allergies to any excipients (such as lactose intolerance), check the specific product information leaflet supplied with your medication or consult your pharmacist before taking doxycycline.