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 aminoacyl-tRNA attachment. It is effective against a wide range of Gram-positive and Gram-negative bacteria, spirochetes, mycoplasmas, chlamydiae, and rickettsiae. In the Indian context, it is a critical, cost-effective antibiotic for managing various bacterial infections, though its use is declining due to resistance and the availability of newer tetracyclines.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are achieved in 2-4 hours after oral administration. | Approximately 6-12 hours, supporting a typical dosing schedule of every 6 hours. | Approximately 75-80% when taken on an empty stomach. Significantly reduced (≤50%) with food, dairy, or divalent cations. |
2. Mechanism of Action
Tetracycline reversibly binds to the 30S subunit of the bacterial ribosome at a position that blocks the binding of aminoacyl-tRNA to the acceptor site on the mRNA-ribosome complex. This inhibition prevents the addition of new amino acids to the growing peptide chain, thereby halting protein synthesis.
3. Indications & Uses
- Acne vulgaris (moderate to severe)
- Brucellosis (in combination with streptomycin or rifampin)
- Chlamydial infections (urethritis, cervicitis, trachoma)
- Rickettsial infections (typhus fever, Rocky Mountain spotted fever)
- Mycoplasma pneumoniae pneumonia
- Helicobacter pylori eradication (as part of combination therapy)
- Chronic bronchitis exacerbations
- Lyme disease (early stage, alternative to doxycycline)
4. Dosage & Administration
Adult Dosage: 500 mg orally every 6 hours. For severe infections, initial dose may be 500 mg every 6 hours, reduced to 250 mg every 6 hours or 500 mg every 12 hours based on response. For acne: 500 mg twice daily, tapering as condition improves.
Administration: Take on an empty stomach, at least 1 hour before or 2 hours after meals. Take with a full glass of water (at least 200 mL) while standing or sitting upright to prevent esophageal irritation and ulceration. Do NOT take with dairy products, antacids, calcium, iron, magnesium, zinc supplements, or bismuth subsalicylate. Maintain at least a 2-3 hour gap.
5. Side Effects
Common side effects may include:
- Nausea
- Vomiting
- Diarrhea
- Epigastric discomfort
- Photosensitivity reaction (sunburn-like rash)
- Discoloration of teeth (in children, fetuses)
- Glossitis (inflamed tongue)
- Stomatitis (mouth sores)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Antacids (Aluminum, Magnesium, Calcium) | Markedly decreased tetracycline absorption due to chelation. | Major |
| Iron Supplements/Salts | Markedly decreased absorption of both agents. | Major |
| Warfarin | Increased anticoagulant effect; risk of bleeding. | Major |
| Oral Contraceptives (Estrogen-containing) | Possible decreased contraceptive efficacy; risk of breakthrough bleeding. | Moderate |
| Methoxyflurane | Increased risk of fatal nephrotoxicity. | Contraindicated |
| Retinoids (Isotretinoin, Acitretin) | Additive risk of pseudotumor cerebri. | Major |
| Digoxin | Increased digoxin bioavailability, risk of toxicity. | Moderate |
| Penicillins (e.g., Amoxicillin) | Antagonistic antibacterial effect; avoid concurrent use. | 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 upright.
- DO complete the full prescribed course, even if you feel better.
- DO use sunscreen and protective clothing to avoid sun exposure due to photosensitivity risk.
- DON'T take with milk, yogurt, cheese, or calcium-fortified juices.
- DON'T take with antacids, iron, calcium, or magnesium supplements (separate by 2-3 hours).
- DON'T give to children under 8 years or use if pregnant/breastfeeding.
- DON'T use outdated medication (can cause Fanconi syndrome).
8. Toxicology & Storage
Overdose: Nausea, vomiting, diarrhea, epigastric pain, dizziness. Large overdoses may lead to pancreatitis, hepatotoxicity, or azotemia in patients with renal compromise.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep in the original container, tightly closed. Keep out of reach of children. Do not use after the expiry date. Do not flush or pour medicines down the drain.