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. Only minocycline is significantly metabolized in the liver. Haemodialysis may be effective in slowly removing tetracyclines, with the exception of doxycycline. Peritoneal dialysis is not effective. They should only be used to treat susceptible strains of these infections when other appropriate anti-infectives are ineffective or contraindicated.