Doxycycline

Doxycycline is a broad-spectrum antibiotic belonging to the tetracycline class. It was first introduced in the 1960s and remains widely used due to its extensive efficacy against a broad range of bacterial pathogens. Its favourable pharmacokinetic properties, including a long half-life and excellent tissue penetration, contribute to its convenience and effectiveness in clinical use. Notably, Doxycycline is well absorbed orally and reaches therapeutic concentrations in various tissues, including those infected by intracellular organisms. These attributes make Doxycycline a mainstay in both primary care and specialist infectious disease management.

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Uses

Doxycycline is employed in the treatment of numerous infections across different systems. It is effective in managing respiratory infections such as pneumonia, bronchitis, and sinusitis, and genitourinary infections like chlamydia and gonorrhoea. Doxycycline is frequently prescribed for acne and rosacea due to its anti-inflammatory properties. Additionally, it is the first-line treatment for many vector-borne diseases, including Lyme disease, Q fever, and rickettsial infections. Doxycycline also plays a significant role in malaria prophylaxis, particularly for travellers visiting endemic regions, and is effective against zoonoses such as leptospirosis and brucellosis.

Dosage and Administration

For most adult infections, Doxycycline is administered at 100 mg once or twice daily. An initial loading dose of 200 mg may be required for acute infections. In malaria prevention, the dose is 100 mg daily, starting 1–2 days before travel and continuing for 4 weeks post-return. For acne, long-term therapy may involve 50–100 mg daily. It is recommended to take Doxycycline with a full glass of water while upright to reduce the risk of oesophageal irritation. Food has minimal effect on its absorption, though it can be taken with meals to lessen gastrointestinal discomfort.

Dose Adjustment in Different Diseases

Renal Impairment

Unlike other tetracyclines, Doxycycline is primarily excreted via the faeces, making it safe for use in patients with mild to moderate renal impairment without dose modification. However, caution should be exercised in end-stage renal disease, and renal function should be monitored in prolonged therapy.

Hepatic Impairment

Because Doxycycline undergoes hepatic metabolism, patients with liver disease should be carefully monitored. Although hepatotoxicity is rare, liver enzymes should be periodically assessed, especially in long-term use. Using the lowest effective dose is advisable in this population.

Pregnancy

Doxycycline is contraindicated during pregnancy due to its potential to impair fetal bone development and cause permanent tooth discolouration. Safer alternatives, such as beta-lactam antibiotics, should be considered in pregnant patients requiring antimicrobial therapy.

Effects and Side Effects

Common side effects of Doxycycline include gastrointestinal symptoms like nausea, vomiting, and abdominal discomfort. Photosensitivity is another notable adverse effect, so sun protection is recommended during treatment. Oesophageal irritation or ulceration may occur if the drug is taken without adequate water or immediately before lying down. Rare but serious reactions include hepatotoxicity, pseudotumour cerebri, blood dyscrasias, and hypersensitivity. Any signs of visual disturbances or hepatic dysfunction require immediate medical evaluation.

How Doxycycline Works

Doxycycline exerts its antibacterial effect by binding to the 30S ribosomal subunit, inhibiting the attachment of aminoacyl-tRNA, and thereby blocking bacterial protein synthesis. It acts as a bacteriostatic agent, preventing bacterial growth rather than directly killing bacteria. Its broad-spectrum activity encompasses Gram-positive and Gram-negative organisms, as well as atypical pathogens such as Mycoplasma, Chlamydia, and Rickettsia.

Drug Combinations in Use

Doxycycline is often used in combination therapy. In brucellosis, it is paired with rifampicin; in malaria, with quinine or artesunate; and in multidrug-resistant tuberculosis, as part of salvage regimens. Intravenous formulations require dilution and should be infused over 60 minutes to prevent local irritation. It should not be mixed with calcium or magnesium-containing fluids, which may impair efficacy.

Presentation or Form

Form	Strength	Dosage Recommendation
Capsule	100 mg	1–2 times daily
Tablet	100 mg	1–2 times daily
IV Injection	100 mg/vial	Administered via slow infusion
Oral Suspension	25 mg/5ml	Typically for paediatric use
Modified-Release Tab	75 mg, 150 mg	Once daily for acne or rosacea

Pharmacokinetics and Pharmacodynamics

Doxycycline has high oral bioavailability (>90%) and reaches peak plasma levels within 1–2 hours. Its half-life ranges from 18 to 22 hours, enabling once or twice daily dosing. The drug is highly protein-bound and extensively distributed in tissues. Its bacteriostatic action is time-dependent, targeting the 30S ribosomal subunit. It has activity against pathogens including Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, Haemophilus influenzae, and Chlamydia trachomatis.

Drug Interactions

Doxycycline absorption is reduced when taken with calcium, magnesium, aluminium, or iron supplements. It may enhance the anticoagulant effect of warfarin and increase the risk of intracranial hypertension when used with isotretinoin. Although its effect on oral contraceptive efficacy is uncertain, additional contraceptive measures are advised.

Comparison with Other Drugs in Same Category

Drug	Spectrum	Half-life	Renal Adjustment	Pregnancy Safety	Tolerability
Doxycycline	Broad-spectrum	18–22 hours	Not required	Contraindicated	Well tolerated
Tetracycline	Narrower	6–12 hours	Required	Contraindicated	Moderate GI upset
Minocycline	Broad-spectrum	11–22 hours	Partial	Contraindicated	Vestibular side effects

Precautions and Special Considerations

• Avoid excessive sunlight; use protective measures.

• Take with adequate water and remain upright post-ingestion.

• Not for use in children <8 years due to dental and skeletal risks.

• Monitor liver function in long-term use.

• Consider alternative contraception.

• Discontinue if signs of hypersensitivity or pseudotumour cerebri appear.

Toxicity or Overdose and Antidote

Overdose may result in gastrointestinal upset, hepatotoxicity, and raised intracranial pressure. There is no specific antidote; treatment involves supportive care and monitoring. Activated charcoal may help in recent ingestions. Severe cases may require hospital admission.

Recent Updates in 2025 and Guidelines

In 2025, NICE reaffirmed Doxycycline as a key treatment for community-acquired pneumonia and acne. WHO continues to include it on the Essential Medicines List. New evidence suggests a role in managing resistant infections and potential applications in chronic inflammatory conditions. It is also being explored for anti-inflammatory effects in viral infections.

Facts to Remember

• Excellent oral absorption and tissue distribution.

• No renal dose adjustment needed.

• Avoid during pregnancy and early childhood.

• Photosensitivity is common—advise protection.

• Potential drug interactions with divalent cations and anticoagulants.

Did You Know?

Doxycycline is under investigation for its effects on neurodegenerative and inflammatory diseases. Its ability to inhibit matrix metalloproteinases offers promise beyond infectious disease treatment.

Mechanism of Resistance

Resistance to Doxycycline occurs via several bacterial mechanisms, including efflux pumps that expel the drug, ribosomal protection proteins that prevent binding to the 30S subunit, and enzymatic degradation. Resistance is increasing in some Gram-negative pathogens, necessitating judicious prescribing and adherence to stewardship principles.

How to Store

• Store at room temperature (15–25°C).

• Protect from light and moisture.

• Keep in original packaging until use.

• Do not use past the expiration date—degraded products may cause toxicity.

References

1. National Institute for Health and Care Excellence (NICE). Doxycycline Guidelines. 2025.

2. World Health Organization (WHO). Model List of Essential Medicines, 2025 Update.

3. British National Formulary (BNF), 2025 Edition.

4. Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 14th Edition.

5. CDC Travel Health: Malaria Prophylaxis Guidelines, 2025.

6. Clinical Infectious Diseases Journal, Volume 72, 2025.

7. Lancet Infectious Diseases, Emerging Resistance Patterns 2025.

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