The body has another way of deactivating drugs in the body. This method of elimination is metabolism or biotransformation. Metabolic processes, in general, have the overall effect of converting drug molecules into more polar compounds. Again, in general, the effect of this should be to decrease tubular re-absorption in the kidney and thus increase drug elimination.
Metabolism takes place by enzymatic catalysis.
Most metabolism occurs in the liver although other sites have been described, such as intestinal wall, kidney, skin, blood.