Intramuscular promethazine and its efficacy in the treatment of Space Motion Sickness (SMS) were evaluated using standardized questions administered during postflight debriefings to crewmembers immediately after their first Shuttle flight. Space Motion Sickness was graded as none, mild, moderate, or severe, based on published criteria. Immediate symptom relief (within 1-2 h) was evaluated by subjective reports; medication efficacy was based on scores derived from the four most frequently reported symptoms of SMS: nausea, vomiting, loss of appetite, and stomach awareness. Scores were given for each symptom, mild = 1, moderate = 2, and severe = 3, and added for a total score for each flight day. Following intramuscular (IM) promethazine on flight day 1, the scores were used to determine if the crewmembers were "sick" or "not sick" on flight day 2. On the basis of the scoring criteria, any subject with a score adding to greater than three, with any severe symptom, or with vomiting was defined as "sick." The comparison showed that 25% of crewmembers treated with IM promethazine were "sick" on flight day 2, compared to 50% of crewmembers who did not receive promethazine (p = 0.046). Of crewmembers treated with IM promethazine, 90% reported immediate symptom relief as well. Untreated crewmembers typically have slow symptom resolution over 72-96 h, and those treated with oral scopolamine/dextroamphetamine show delayed symptom development. This study suggests that intramuscular promethazine is an effective treatment for SMS and merits continued use and further controlled investigations.