Doxycycline

Adults: 100 mg twice daily on day 1, then 100 mg once or twice daily. Malaria prophylaxis: 100 mg daily starting 1–2 days before travel.

Photosensitivity, esophageal irritation (take upright with water), GI upset, vaginal candidiasis.

Pregnancy, breastfeeding, children <8 years (permanent tooth discoloration), tetracycline allergy.

Doxycycline inhibits bacterial protein synthesis at the 30S ribosome. Effective against atypical respiratory pathogens (Mycoplasma, Chlamydia), tick-borne disease, severe acne, and as malaria prophylaxis when traveling to endemic regions.