Doxycycline is a semi-synthetic tetracycline derived from oxytetracycline. It is active against Gram-positive and Gram-negative bacteria. It accumulates in the liver, kidney, bones and intestine. It reaches higher concentrations in the lungs than in plasma, and gives a concentration in synovial fluid 4.6 times greater than that in plamsa. Karidox is marketed in partnership with Vetsonic LLP.