Doxycycline, developed by Pfizer in the early 1960s, was created by modifying oxytetracycline to produce a more stable and effective antibiotic. It was first approved by the FDA in 1967 under the brand name Vibramycin and became Pfizer’s first once-daily broad-spectrum antibiotic. Over time, its uses have expanded, including malaria prophylaxis since the 1990s. Doxycycline is a broad-spectrum tetracycline antibiotic that treats a wide range of bacterial infections by inhibiting bacterial protein synthesis, thereby preventing their growth and reproduction.
BRAND NAMES
Doxycycline is marketed under several brand names, including Vibramycin®, Doryx®, Oracea®, Monodox®, and Periostat®. Generic versions are widely available, providing more affordable alternatives. Some formulations are designed specifically for certain conditions, like Oracea for rosacea.
MECHANISM OF ACTION
Doxycycline is a broad-spectrum tetracycline antibiotic that works by inhibiting bacterial protein synthesis. It binds to the 30S ribosomal subunit, preventing aminoacyl-tRNA from attaching to the ribosome, which stops bacterial growth. This bacteriostatic effect makes it effective against Gram-positive, Gram-negative, and atypical bacteria. Its inhibition of protein synthesis prevents bacterial replication without directly killing the organisms. This mechanism underlies its effectiveness against a wide range of infections.
PHARMACOKINETICS:
Absorption
Doxycycline is well absorbed when taken orally, with peak plasma concentrations reached within 2–4 hours. It has high oral bioavailability, usually over 90%, and is available in both immediate and delayed-release formulations.
Distribution
The drug is widely distributed in body tissues, including the lungs, liver, and kidneys, and crosses the placenta while being present in breast milk. It binds moderately to plasma proteins (around 90–95%) and has a volume of distribution of 0.7–1.4 L/kg. This broad tissue penetration makes doxycycline effective for systemic infections.
Metabolism
The drug is widely distributed in body tissues, including the lungs, liver, and kidneys, and crosses the placenta while being present in breast milk. It binds moderately to plasma proteins (around 90–95%) and has a volume of distribution of 0.7–1.4 L/kg. This broad tissue penetration makes doxycycline effective for systemic infections.
Excretion
The drug is primarily excreted in feces, with a smaller portion eliminated via urine. Renal impairment has minimal effect on clearance. The elimination half-life is approximately 18–22 hours, allowing for convenient dosing while maintaining effective therapeutic levels.
PHARMACODYNAMICS
Doxycycline exhibits a bacteriostatic effect by inhibiting bacterial protein synthesis, preventing replication. It is effective against a wide range of pathogens, including respiratory, urinary, and sexually transmitted infections. Additionally, it has anti-inflammatory properties, useful in treating acne and rosacea. Its long half-life supports convenient dosing schedules. Broad tissue penetration ensures effectiveness against infections in multiple organ systems.
ADMINISTRATION
Doxycycline is administered orally as capsules, tablets, or oral suspension, and intravenously in severe infections. Standard dosing is once or twice daily, usually taken with a full glass of water to prevent esophageal irritation. Patients are advised to remain upright for at least 30 minutes after oral ingestion. Administration schedules may vary depending on the type and severity of infection. Careful adherence to dosing instructions ensures optimal therapeutic outcomes.
DOSAGE AND STRENGTH
Doxycycline is commonly available in strengths of 50 mg, 75 mg, 100 mg, and 150 mg tablets or capsules. The usual adult dose for bacterial infections is 100 mg twice daily on the first day, followed by 100 mg once daily thereafter. For severe infections, the dose may be increased to 100 mg twice daily. In children above 8 years and weighing less than 45 kg, dosing is based on body weight. The medication is also available as an oral suspension and injectable form for hospital use.
DRUG INTERACTIONS
Doxycycline interacts with several drugs that may affect its absorption or efficacy. Antacids, iron supplements, and calcium-containing products can reduce doxycycline absorption by forming insoluble complexes. It may enhance the effect of warfarin, increasing bleeding risk. The drug’s effectiveness can be decreased by enzyme-inducing drugs such as phenytoin, barbiturates, or carbamazepine. Concurrent use with retinoids can increase the risk of intracranial hypertension.
FOOD INTERACTIONS
Doxycycline can be taken with or without food, but taking it with food or milk helps reduce stomach upset. However, dairy products may slightly lower its absorption due to calcium binding. To optimize absorption, it is best taken with a full glass of water and to remain upright for at least 30 minutes afterward. Avoid taking it with iron, magnesium, or aluminum supplements near the dosing time. Alcohol consumption can also reduce its effectiveness in long-term use.
CONTRAINDICATIONS
Doxycycline is contraindicated in individuals with known hypersensitivity to tetracyclines. It should not be used in children under 8 years because it can cause permanent discoloration of teeth and affect bone growth. Pregnant and breastfeeding women are advised to avoid it due to possible harm to the fetus or infant. It is also contraindicated in patients with severe liver impairment. Use caution in those with esophageal disorders, as the drug can cause irritation or ulcers.
SIDE EFFECTS
Loss of appetite
Loose stools or diarrhea
Trouble or pain while swallowing
Headache or feeling of pressure in the head
Elevated blood urea nitrogen (BUN) levels
Increased sensitivity of the skin to sunlight (photosensitivity)
Feeling nauseous
Development of a skin rash
Temporary discoloration of teeth, which usually resolves after professional dental cleaning once the medication is discontinued
Inflammation or soreness of the tongue
Vomiting or upset stomach
OVER DOSE
An overdose of doxycycline is uncommon but can occur if taken in large amounts. Symptoms may include nausea, vomiting, dizziness, and stomach pain. In severe cases, it may cause liver toxicity or renal impairment. Treatment is mainly supportive, involving gastric lavage and administration of activated charcoal if recent ingestion occurred. Patients should seek immediate medical care for monitoring and symptom management.
TOXICITY
Doxycycline has relatively low toxicity, but prolonged or high-dose use can lead to complications. Chronic use may result in hepatotoxicity or photosensitivity reactions, where the skin becomes overly sensitive to sunlight. Long-term therapy can also alter normal gut flora, leading to superinfections such as candidiasis. In rare cases, intracranial hypertension has been reported, presenting with headaches and vision problems. Toxic effects usually resolve after stopping the drug.
