Drug distribution typically refers to the movement of the drug to and fro from the blood to surrounding tissues and its relative proportions in tissues. After absorption into the bloodstream, drugs rapidly circulate throughout the body to various tissues such as fat, muscle, and brain tissues. Drugs that dissolve in water potentially stay in the bloodstream and the interstitial space for a more extended period. These drugs have a low probability of crossing the membranes in the tissues. For instance, antibiotic penicillin does not readily cross the blood-brain barrier.