Microgravity and drug effects on bone.

Microgravity changes the metabolic environment of bone leading to site-specific alterations in bone remodeling: bone formation is decreased and bone resorption is increased resulting in significant bone loss with an attendant increase in fracture risk. The pelvis and lower extremities show the greatest loss of bone that may amount to as much as 5% in the lumbar spine and 10% in the proximal femur after spaceflight of 6 months duration 1 . Multiple drugs have been employed during manned spaceflight to combat a variety of flight-related medical disorders such as space motion sickness, sleeplessness, nasal congestion, headache, back pain and constipation. The list includes antibiotics, sleep medications, drugs for nasal congestion, antiarrythmics, and muscle relaxants 2 . However, to date, only exercise has been employed to counter bone loss: the effects of different regimens have not proven protective of bone. It is appreciated that the response of bone to drugs will be different in extended spaceflight as compared to responses on the ground 3 . Extended bed rest studies on earth have not focused on pharmacodynamics of bone active drugs. The few reported observations that involving drugs such as acetaminophen and promethazine indicate that microgravity has potentially profound effects on drug actions when compared to earth 4 . Furthermore, drug action in microgravity may be affected by factors such as gender, which is not considered a primary fact when assessing drug actions on earth, but which may assume greater importance in microgravity 5 .