Doxycycline is a broad-spectrum, second-generation, long-acting tetracycline-class antibiotic. It is a semi-synthetic derivative of oxytetracycline. It is bacteriostatic and exerts its effect by inhibiting protein synthesis in susceptible bacteria. It is widely used in India for a variety of bacterial infections, including respiratory tract infections, acne, sexually transmitted infections, and prophylaxis for malaria. Its favorable pharmacokinetic profile, including good oral bioavailability and long half-life allowing once or twice-daily dosing, makes it a preferred tetracycline in clinical practice.
Adult: For most infections: 100 mg twice daily on the first day (loading dose), followed by 100 mg once daily. For severe infections: 100 mg twice daily continued. For acne: 50-100 mg once daily. For malaria prophylaxis: 100 mg once daily, starting 1-2 days before travel, during travel, and for 4 weeks after leaving the endemic area.
Note: Take with a full glass of water while sitting or standing upright to prevent esophageal irritation and ulceration. Preferably take on an empty stomach (1 hour before or 2 hours after meals) for optimal absorption. If gastrointestinal upset occurs, it may be taken with food (non-dairy), but absorption may be slightly reduced. Avoid concomitant intake with dairy products, antacids, calcium, iron, magnesium, aluminum, zinc, or bismuth subsalicylate; separate administration by at least 2-3 hours.
Doxycycline binds reversibly to the 30S ribosomal subunit of susceptible bacteria, preventing the binding of aminoacyl-tRNA to the acceptor (A) site on the mRNA-ribosome complex. This inhibits the addition of new amino acids to the growing peptide chain, thereby inhibiting protein synthesis and exerting a bacteriostatic effect.
Pregnancy: Category D. Contraindicated, especially in the second and third trimesters. Can cause permanent discoloration of the fetus's teeth (yellow-gray-brown) and reversible inhibition of bone growth. Should not be used during pregnancy unless the potential benefit justifies the potential risk to the fetus (e.g., life-threatening infection like Rocky Mountain spotted fever).
Driving: Doxycycline may cause dizziness, lightheadedness, or blurred vision (especially with pseudotumor cerebri). Patients should be cautioned about operating machinery or driving until they are sure they are not affected.
| Antacids (Aluminum, Calcium, Magnesium) | Markedly decreased doxycycline absorption due to chelation | Major |
| Iron supplements and multivitamins with iron | Decreased doxycycline absorption due to chelation | Major |
| Warfarin | Increased anticoagulant effect; increased risk of bleeding | Major |
| Oral Contraceptives (Estrogen-containing) | Possible decreased contraceptive efficacy; risk of breakthrough bleeding | Moderate |
| Penicillins (e.g., Amoxicillin) | Potential antagonism of bactericidal activity of penicillins | Moderate |
| Retinoids (Isotretinoin, Acitretin) | Additive risk of benign intracranial hypertension | Major |
| Anticonvulsants (Carbamazepine, Phenytoin, Barbiturates) | Increased metabolism of doxycycline, reducing its half-life and efficacy | Moderate |
| Methoxyflurane | Increased risk of fatal renal toxicity | Contraindicated |