DOXYCYCLINE HYCLATE
Clinical safety rating: avoid
Antacids and calcium supplements decrease absorption Can cause photosensitivity and esophageal irritation.
Doxycycline hyclate is a bacteriostatic tetracycline antibiotic that 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-angiogenic properties.
| Metabolism | Doxycycline is metabolized partially in the liver via glucuronidation and non-enzymatic pathways. It is not significantly metabolized by cytochrome P450 enzymes. Approximately 40% is excreted unchanged in urine via glomerular filtration, and the remainder is eliminated in feces via biliary secretion. |
| Excretion | Approximately 40% excreted unchanged in urine via glomerular filtration; 20-30% eliminated in feces via biliary secretion and nonbiliary routes; the remainder is metabolized. Enterohepatic circulation contributes to prolonged half-life. |
| Half-life | 18-24 hours in patients with normal renal function; may increase to 24-48 hours in renal impairment; clinical context: allows once- or twice-daily dosing. |
| Protein binding | 80-93% bound primarily to plasma proteins, especially albumin. |
| Volume of Distribution | 0.75-1.3 L/kg; indicates extensive tissue penetration including bone, teeth, and respiratory tract. |
| Bioavailability | Oral: 90-100% (doxycycline hyclate); IV: 100%. |
| Onset of Action | Oral: 1-2 hours (peak serum levels); IV: immediate; topical: 1-2 weeks for clinical improvement. |
| Duration of Action | Antimicrobial effect persists for 12-24 hours after a single dose; clinical response in infections typically seen within 24-48 hours. |
| Molecular Weight | 1025.89 Da |
100 mg orally or intravenously every 12 hours on day 1, then 100 mg daily. For severe infections or certain indications, 100 mg every 12 hours.
| Dosage form | TABLET |
| Renal impairment | No dosage adjustment required for mild to moderate renal impairment. For severe renal impairment (CrCl <10 mL/min), use with caution and consider extending dosing interval to every 24 hours. Not significantly removed by hemodialysis. |
| Liver impairment | No specific Child-Pugh based adjustments; use with caution in severe hepatic impairment due to potential hepatotoxicity. Monitor liver function. |
| Pediatric use | For children >8 years and weighing ≤45 kg: 2.2 mg/kg orally or intravenously every 12 hours on day 1, then 2.2 mg/kg once daily; for severe infections, 2.2 mg/kg every 12 hours. For children >45 kg: use adult dose. Not recommended in children <8 years due to tooth discoloration. |
| Geriatric use | No specific dose adjustment required; use lower end of dosing range due to age-related decline in renal function and increased risk of photosensitivity and esophageal irritation. Ensure adequate hydration. |
| 1st trimester | Avoid; associated with fetal toxicity and teratogenicity, including inhibition of bone growth and tooth discoloration. |
| 2nd trimester | Avoid; risk of permanent tooth discoloration and bone development suppression. |
| 3rd trimester | Avoid; risk of permanent tooth discoloration and bone development suppression. |
Clinical note
Antacids and calcium supplements decrease absorption Can cause photosensitivity and esophageal irritation.
| FDA category | Positive |
| Placental transfer | Doxycycline crosses the placenta extensively, achieving fetal serum concentrations approximately 60-75% of maternal levels. Binding to fetal tissues, especially bones and teeth, occurs. |
| Breastfeeding |
■ FDA Black Box Warning
None.
| Common Effects | Photosensitivity |
| Serious Effects |
Hypersensitivity to doxycycline or any tetracyclinePregnancy (especially second and third trimesters)Children under 8 years of age (risk of permanent tooth discoloration and bone growth suppression)
| Precautions | Photosensitivity: Avoid prolonged sun exposure; use sunscreen., Gastrointestinal irritation: Take with food or milk to reduce risk of esophagitis; do not lie down immediately after dosing., Hepatotoxicity: Use with caution in patients with hepatic impairment., Tooth discoloration: Avoid use in children <8 years old unless no alternative; may cause permanent yellow-brown-gray discoloration., Bone growth impairment: Avoid use in pregnant or lactating women as it may affect fetal/infant bone development., Superinfection: May lead to overgrowth of non-susceptible organisms (e.g., Clostridioides difficile)., Increased intracranial pressure: Can cause pseudotumor cerebri (usually reversible upon discontinuation)., Use in pregnancy: Category D; avoid unless necessary for anthrax or other serious infections. |
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| Doxycycline is excreted into breast milk in low concentrations. However, due to the theoretical risk of dental staining and bone growth inhibition in the nursing infant, alternative antibiotics are preferred. If use is necessary, the benefit must outweigh the risk. |
| Lactation Rating | L3 - Moderately Safe |
| Teratogenic Risk | Doxycycline is classified as FDA pregnancy category D. It crosses the placenta. First trimester: Avoid due to risk of teratogenicity (possible neural tube defects, cardiovascular malformations) based on animal studies, but human data are limited. Second and third trimesters: Use only if no alternative; associated with reversible inhibition of fetal bone growth and permanent yellow-brown discoloration of deciduous teeth (enamel hypoplasia) due to calcium chelation. Theoretical risk of maternal hepatotoxicity. |
| Fetal Monitoring | Monitor liver function tests (LFTs) and renal function periodically due to potential hepatotoxicity. In third trimester, monitor fetal growth and development (ultrasound). Assess infant for signs of bone growth impairment or dental discoloration after birth. If used for prolonged periods, consider monitoring for pseudotumor cerebri in mother and infant. |
| Fertility Effects | No known direct effects on fertility in humans. Animal studies have shown no impairment of fertility at therapeutic doses. However, doxycycline may affect male reproductive function through effects on sperm motility, but clinical significance is unclear. |
| Food/Dietary | Dairy products (milk, cheese, yogurt) and calcium-fortified foods can chelate doxycycline, reducing absorption. Separate ingestion by at least 2 hours. High-fat meals may slightly decrease absorption but are less significant than divalent cations. Alcohol does not interact directly but may exacerbate GI side effects. |
| Clinical Pearls | Doxycycline hyclate is highly bioavailable and can be administered with food to reduce GI upset, though absorption is slightly decreased. It is the preferred tetracycline in renal impairment due to non-renal clearance. Avoid in children <8 years and pregnancy (pregnancy category D). Photosensitivity is common; counsel on sun avoidance. Rapid intravenous infusion may cause thrombophlebitis. Due to its anti-inflammatory and anti-angiogenic properties, it is used off-label for rosacea and as adjunct in some cancers. |
| Patient Advice | Take with a full glass of water to avoid esophageal irritation. Do not lie down for at least 30 minutes after taking. · Avoid taking with antacids, iron, calcium, magnesium, or bismuth subsalicylate. Separate by 2-3 hours. · Avoid prolonged sun exposure and use sunscreen; severe sunburn can occur even with brief exposure. · Complete the full course even if you feel better to prevent resistance. · This drug may reduce effectiveness of oral contraceptives; use backup contraception. · Notify your doctor if you develop diarrhea, skin rash, or vaginal itching. · Store at room temperature away from light and moisture. |