Doxycycline

Doxycycline is a broad-spectrum antibiotic in the tetracycline class. It works by reversibly binding to the 30S ribosomal subunit in bacteria, blocking the attachment of aminoacyl-tRNA and inhibiting protein synthesis. It is active against many Gram-positive and Gram-negative bacteria as well as atypical pathogens such as Mycoplasma, Chlamydiae, and Rickettsiae. It is widely used for a range of infections and some noninfectious indications.

Pharmacokinetics and dosing notes: doxycycline is well absorbed orally and can be taken with or without food, though calcium, iron, and other divalent cations can reduce absorption. It has a long half-life, allowing once-daily dosing in many regimens. Excretion occurs in both feces and urine, with a substantial portion cleared hepatically; dose adjustments are not usually required in mild to moderate renal impairment.

Clinical uses: it is employed for respiratory and atypical pneumonias, tick-borne and other rickettsial diseases, chlamydial infections, and certain sexually transmitted infections. It is also used for acne and rosacea, and as malaria chemoprophylaxis in regions with chloroquine-resistant Plasmodium falciparum. In some regimens, doxycycline is part of Helicobacter pylori eradication therapy.

Safety and cautions: tetracyclines are generally avoided during pregnancy and in children under eight due to risks of tooth discoloration and enamel hypoplasia. Photosensitivity is a common adverse effect. Esophageal irritation or ulcers can occur if tablets are not taken with sufficient water. Caution is advised in severe liver disease.

Interactions and resistance: absorption can be reduced by concomitant calcium, magnesium, aluminum, iron, and zinc. Bacterial resistance via efflux pumps and ribosomal protection proteins has increased in some species.