Minocycline is a semi-synthetic tetracycline antibiotic that has many similarities with doxycycline. Minocycline may be used orally (dogs, cats and horses) or intravenously (dogs and cats). Tetracycline antibiotics are broad spectrum and bacteriostatic. Their mechanism of action is through the inhibition of protein synthesis and the alteration of cytoplasmic membrane permeability within the susceptible organism. Minocycline has increased penetration into bacterial cells and increased pharmacologic activity when compared to tetracycline. It is effective against a broad spectrum of gram negative and gram positive bacteria, and a wide variety of other organisms including Rickettsia, Spirochetes, Mycoplasma, and Chlamydia. In addition to its use as an antibiotic, minocycline is used in low doses as an anti-inflammatory and adjunct treatment for arthritis and degenerative joint disease in dogs and horses. This use is based on the inhibition of matrix metalloproteinases. Minocycline is well absorbed after oral administration, and its absorption is minimally affected by the presence of food in the stomach. It is less disruptive to normal GI bacterial flora than tetracycline. Minocycline may be used in animals with decrease kidney function because it is extensively metabolized via the liver, and then primarily excreted as inactive metabolites in the feces and urine.