Space motion sickness (SMS) is a condition similar to ordinary travel sickness, produced by confusion of the human vestibular (balance) system and the absence of a gravitational vertical. It is also known as microgravity nausea. About one-third of Apollo crewmembers reported some level of SMS, rising to 60% on the longer-duration Skylab missions. During Space Shuttle flights the incidence of SMS in first-time astronauts has also been about 60%. Treatment with such medications as intramuscular promethazine (Phenergan) has been effective, and the nausea tends to wear off two to three days into the flight.