DOXYCYCLINE
Clinical safety rating: avoid
Antacids and calcium supplements decrease absorption Can cause photosensitivity and esophageal irritation.
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, preventing the attachment of aminoacyl-tRNA to the mRNA-ribosome complex. It also exhibits anti-inflammatory and anti-collagenase activities.
| Metabolism | Doxycycline is partially metabolized in the liver via unspecified pathways; it is not significantly metabolized by CYP450 enzymes. Approximately 40% is excreted renally as active drug. |
| Excretion | Renal (40%) and fecal/biliary (60%); undergoes enterohepatic circulation; active drug and metabolites excreted in urine and feces. |
| Half-life | Terminal elimination half-life is 18–24 hours in patients with normal renal function; prolonged to 20–30 hours in renal impairment; allows once or twice daily dosing. |
| Protein binding | 80–93% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | 0.75–1.3 L/kg, indicating extensive tissue penetration; high concentrations in lung, liver, bone, and prostate. |
| Bioavailability | Oral: 90–100% (well absorbed); IV: 100%; topical: minimal systemic absorption (<10%). |
| Onset of Action | Oral: 2–4 hours (therapeutic levels); IV: immediate after administration; topical: 1–2 days for clinical effect. |
| Duration of Action | 12–24 hours due to long half-life; detectable levels for up to 48 hours after last dose; clinical effect persists for 24–48 hours. |
| Molecular Weight | 444.43 |
100 mg orally or intravenously every 12 hours on day 1, then 100 mg every 12 hours or 50 mg every 6 hours.
| Dosage form | CAPSULE |
| Renal impairment | For CrCl < 50 mL/min: no dosage adjustment required for most indications; for severe infections or prolonged use, consider monitoring renal function. In patients with CrCl < 10 mL/min, reduce dose or avoid if possible due to potential anti-anabolic effect. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: use with caution; no specific dose reduction recommended. Child-Pugh C: avoid use due to lack of safety data. |
| Pediatric use | For children >8 years and weighing ≤45 kg: 2.2 mg/kg every 12 hours on day 1, then 2.2 mg/kg once daily or 1.1 mg/kg every 12 hours. For children >45 kg: same as adult. For children <8 years: contraindicated due to risk of permanent tooth discoloration and enamel hypoplasia. |
| Geriatric use | No specific dose adjustment required; use standard adult dosing. Monitor renal function and consider potential increased risk of photosensitivity reactions. Avoid in elderly with impaired renal function if alternative agents available. |
| 1st trimester | Avoid; associated with skeletal abnormalities and teeth discoloration; use only if no alternative. |
| 2nd trimester | Avoid; may cause permanent teeth discoloration and enamel hypoplasia; use only if no alternative. |
| 3rd trimester | Avoid; can cause fetal skeletal retardation and teeth discoloration; use only if no alternative. |
Clinical note
Antacids and calcium supplements decrease absorption Can cause photosensitivity and esophageal irritation.
| FDA category | Positive |
| Placental transfer | Crosses placenta; achieves fetal serum concentrations 60-70% of maternal levels. |
| Breastfeeding |
■ FDA Black Box Warning
There is no FDA black box warning for doxycycline.
| Common Effects | Photosensitivity |
| Serious Effects |
Hypersensitivity to doxycycline or any tetracyclineConcomitant use with isotretinoinPregnancy (especially second and third trimesters)Lactation (prolonged use)Children under 8 years of age (risk of permanent tooth discoloration)
| Precautions | Photosensitivity: avoid prolonged sun exposure, Esophageal injury: take with adequate fluids, Hepatotoxicity: caution in hepatic impairment, Intracranial hypertension: risk of pseudotumor cerebri, Delay in bone growth and tooth discoloration in children <8 years, C. difficile-associated diarrhea, Superinfection with resistant organisms |
| Food/Dietary | Dairy products (milk, cheese, yogurt), calcium-fortified foods, antacids containing aluminum, calcium, magnesium, and iron supplements can chelate doxycycline, reducing absorption. Separate intake by at least 2 hours. Alcohol is not known to interact significantly. Avoid taking with high-iron foods like spinach or red meat within 2 hours. |
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| Excreted into breast milk in low amounts; theoretical risk of teeth discoloration and bone growth suppression in nursing infants. Use with caution, especially with prolonged use. |
| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | Category D. Avoid in pregnancy. Risk of fetal harm including permanent tooth discoloration and impaired bone growth when used in second and third trimesters. First trimester use associated with neural tube defects, cardiovascular malformations, and spontaneous abortion. Hepatic necrosis in pregnant women reported. |
| Fetal Monitoring | Monitor liver function tests, renal function, and complete blood count periodically due to potential hepatotoxicity and blood dyscrasias. For prolonged use, monitor thyroid function. In pregnancy, monitor fetal growth and development via ultrasound if inadvertent exposure. |
| Fertility Effects | No known adverse effects on human fertility. Animal studies show reversible effects on spermatogenesis at high doses; clinical significance in humans minimal. |
| Clinical Pearls | Administer with a full glass of water to reduce esophageal irritation; avoid lying down for 30 minutes after dosing. Tetracyclines bind calcium, so avoid dairy, antacids, and iron within 2 hours of dosing. Use sun protection due to photosensitivity. In children under 8, pregnant, or breastfeeding, avoid due to tooth discoloration and bone growth inhibition. Monitor for superinfection, especially Clostridioides difficile. Dose adjustment not needed in renal impairment but caution in hepatic impairment. |
| Patient Advice | Take exactly as prescribed; finish the full course even if you feel better. · Take with a full glass of water and remain upright for 30 minutes after. · Avoid dairy products, antacids, calcium supplements, iron, and magnesium within 2 hours of taking doxycycline. · Use sunscreen and protective clothing; avoid prolonged sun exposure as it can cause severe sunburn. · Do not take if pregnant, breastfeeding, or if you have a child under 8 years old. · Report any signs of severe diarrhea, skin rash, or difficulty swallowing to your doctor. · Store at room temperature away from moisture and heat; do not use outdated medication. |