Adults

Typically 100 mg once or twice daily, depending on the infection. For severe infections, initial dose 200 mg, then 100 mg daily.

Elderly

No specific dosage adjustment is generally needed, but monitor renal and hepatic function, especially with prolonged use.

Renal_impairment

No significant dosage adjustment is usually required as doxycycline is primarily excreted non-renally.

Onset

Antibacterial effects typically begin within a few hours of administration.

Excretion

Primarily excreted unchanged in the feces via biliary excretion, with a smaller amount eliminated in urine.

Half life

Approximately 12-22 hours (average 18 hours), allowing for once or twice daily dosing.

Absorption

Well absorbed orally, nearly completely absorbed from the gastrointestinal tract.

Metabolism

Partially metabolized in the liver, primarily via glucuronidation.

Penicillins

Tetracyclines can interfere with the bactericidal action of penicillins. Avoid concurrent use.

Methotrexate

Concurrent use may increase methotrexate toxicity.

Oral contraceptives

Doxycycline may reduce the effectiveness of oral contraceptives, leading to breakthrough bleeding or unintended pregnancy. Advise alternative birth control methods.

Anticoagulants (e.g., warfarin)

Doxycycline may potentiate the effects of anticoagulants, requiring dosage adjustment.

Antacids containing aluminum, calcium, or magnesium; iron preparations; and bismuth subsalicylate

These can impair the absorption of doxycycline. Administer doxycycline 2-3 hours before or after these agents.