Adults

General dosage: 100 mg twice daily on the first day, followed by 100 mg once daily. For severe infections, 100 mg twice daily. For acne: 50 mg daily. For malaria prophylaxis: 100 mg daily starting 1-2 days before travel, continuing for 4 weeks after return.

Elderly

No specific dose adjustment is generally required based on age alone, but caution should be exercised in elderly patients with significant hepatic impairment.

Renal_impairment

No dosage adjustment is usually necessary for patients with impaired renal function, as doxycycline is primarily eliminated by non-renal routes.

Onset

Therapeutic effects are typically observed within hours to days depending on the infection.

Excretion

Primarily excreted unchanged in feces via non-renal mechanisms (biliary excretion), and to a lesser extent, via urine. This makes it suitable for use in patients with renal impairment.

Half life

Approximately 16-24 hours, allowing for once or twice daily dosing.

Absorption

Well absorbed from the gastrointestinal tract, even in the presence of food or milk, though absorption may be slightly reduced by dairy products, antacids, and iron preparations. Peak plasma concentrations are reached within 2-4 hours.

Metabolism

Partially metabolized in the liver to inactive metabolites. It undergoes enterohepatic recirculation.

Warfarin

Doxycycline may enhance the anticoagulant effect of warfarin. Monitor INR/PT closely.

Penicillins

Tetracyclines can interfere with the bactericidal action of penicillin. Avoid concomitant use.

Oral contraceptives

May reduce the effectiveness of oral contraceptives. Advise use of alternative birth control methods.

Barbiturates, carbamazepine, phenytoin

These drugs can decrease the half-life of doxycycline, leading to reduced efficacy. Increased doxycycline dosage may be required.

Antacids containing aluminum, calcium, or magnesium, and iron preparations

Can significantly impair doxycycline absorption. Administer doxycycline 2-3 hours before or after these agents.