doxycycline UK /ˌdɒk.sɪˈsʌɪ.kliːn/US /ˌdak.siˈsaɪːklin/ noun1 •A broad-spectrum antibiotic C₂₂H₂₄N₂O₈ of the tetracycline class, which has a long half-life in the body and used orally to treat various bacterial infections.When his symptoms didnʼt improve, he was admitted to a local hospital and given doxycycline, a drug for tick-bite infections. — 2019 formsdoxycyclines