1. Clinical Overview
Tetracycline is a broad-spectrum, bacteriostatic antibiotic belonging to the tetracycline class. It inhibits protein synthesis by binding to the 30S ribosomal subunit, preventing the attachment of aminoacyl-tRNA to the ribosomal acceptor site. It is effective against a wide range of Gram-positive and Gram-negative bacteria, as well as atypical organisms like Mycoplasma, Chlamydia, and Rickettsia. In the Indian context, its use has declined due to high resistance rates, but it remains a key drug for specific infections like Rickettsial diseases, brucellosis, and as an alternative for acne vulgaris.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are reached within 2-4 hours after oral administration. | Approximately 6-12 hours, supporting a typical dosing schedule of 250-500 mg every 6 hours. | Approximately 75-80% when taken on an empty stomach. Significantly reduced (up to 50%) with food, milk, or divalent cations. |
2. Mechanism of Action
Tetracycline is a bacteriostatic agent. It reversibly binds to the 30S subunit of the bacterial ribosome, specifically to the A site. This prevents the binding of aminoacyl-tRNA to the mRNA-ribosome complex, thereby inhibiting the addition of new amino acids to the growing peptide chain during protein synthesis.
3. Indications & Uses
- Rickettsial infections (e.g., Typhus fever, Rocky Mountain spotted fever)
- Brucellosis (in combination with streptomycin or gentamicin)
- Chlamydial infections (e.g., Psittacosis, Lymphogranuloma venereum)
- Mycoplasma pneumoniae pneumonia
- Acne vulgaris (moderate to severe, as a second-line agent)
- Cholera
4. Dosage & Administration
Adult Dosage: 250-500 mg orally every 6 hours. For severe infections (e.g., brucellosis): 500 mg every 6 hours. For acne: 250-500 mg twice daily.
Administration: Take on an empty stomach, at least 1 hour before or 2 hours after meals. Take with a full glass of water (240 mL) while sitting or standing to prevent esophageal irritation and ulceration. Do NOT take with dairy products (milk, yogurt, cheese), antacids, calcium, iron, magnesium, zinc, or aluminum-containing products. Separate administration by at least 2-3 hours.
5. Side Effects
Common side effects may include:
- Nausea, vomiting, epigastric distress
- Diarrhea (can be due to Clostridioides difficile)
- Photosensitivity reaction (sunburn-like rash)
- Discoloration of teeth (in children, fetuses)
- Oral or vaginal candidiasis (thrush)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Antacids (Al, Ca, Mg), Iron, Zinc, Bismuth subsalicylate | Chelation reduces tetracycline absorption by >50% | Major |
| Warfarin | Increased anticoagulant effect; risk of bleeding | Major |
| Oral Contraceptives (Estrogen-containing) | Reduced contraceptive efficacy; risk of breakthrough bleeding/pregnancy | Major |
| Methoxyflurane | Increased risk of fatal nephrotoxicity | Contraindicated |
| Retinoids (Isotretinoin, Acitretin) | Additive risk of benign intracranial hypertension | Major |
| Digoxin | Increased digoxin bioavailability (due to gut flora alteration) | Moderate |
| Penicillins (e.g., Amoxicillin) | Antagonistic effect; tetracycline may reduce penicillin efficacy | Moderate |
7. Patient Counselling
- DO take on an empty stomach (1 hr before or 2 hrs after food).
- DO take with a full glass of water while sitting/standing.
- DO complete the full course of therapy, even if you feel better.
- DO use sunscreen and protective clothing to avoid photosensitivity.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DON'T take with dairy products, antacids, calcium, or iron supplements.
- DON'T lie down for at least 30 minutes after taking the dose.
- DON'T give this medicine to children under 8 years old.
8. Toxicology & Storage
Overdose: Nausea, vomiting, diarrhea, epigastric pain, dizziness. In massive overdose, hepatotoxicity (jaundice, elevated LFTs) and azotemia may occur. Esophageal ulceration is possible if capsules lodge in the esophagus.
Storage: Store below 30°C (86°F) in a cool, dry place, protected from light and moisture. Keep the container tightly closed. Keep out of reach of children. Do not use after the expiry date printed on the pack.