More study is needed in order to clearly establish its role in the management of this disease. Resistance to tetracyclines may be natural or acquired and usually results in decreased permeability of the organism's cell wall to tetracycline. Complete cross resistance occurs among members of the tetracycline class, with the exception of minocycline which exhibits only partial cross resistance. Absorption takes place mainly in the stomach and upper small intestine. The presence of food, milk or cations may significantly decrease the extent of absorption.
Tetracyclines are poorly and erratically absorbed after i.m. injection.
Tetracyclines are distributed into most body tissues and fluids. They are distributed into the bile and undergo varying degrees of enterohepatic recirculation. The tetracyclines tend to localize in tumors, necrotic or ischemic tissue, liver and spleen and form tetracycline-calcium orthophosphate complexes at sites of new bone formation or tooth development. Tetracyclines readily cross the placenta and are excreted in high amounts in breast milk. For the most part, the tetracyclines are excreted unchanged in the urine and bile or secreted into the GI tract.