Physiological changes in spaceflight that may affect drug action.
Despite nearly 40 years of spaceflight, the paucity of time spent in microgravity and somewhat scattered approach to the study to pharmacology in microgravity have resulted in little solid information. Moreover, they have generally only been studied in the first few days of spaceflight or its simulation by bedrest, when physiological changes are most dynamic. Nevertheless, early physiological responses to microgravity have been well documented and provide good support for the hypothesis that both pharmacokinetics and pharmacodynamics should be altered, with more significant changes occurring as stays in space increase in duration. Whether given orally, by intravenous (IV) or intramuscular (IM) injection, intranasally (IN) or rectally, a drug must distribute through the body, usually crossing certain membranes and becoming bound to other molecules, reach its receptor and bind to it in order to exert its action. This paper, then, will follow a drug's passage through the body in terms of pharmacokinetic parameters known to be affected by microgravity, and mention other factors.