There is no evidence that astronauts have been drunk on flight missions, an investigation by US space agency Nasa's safety chief has found.
Nasa's safety chief reviewed space flights going back 20 years
His probe was prompted by a report that said Nasa astronauts had been cleared to fly while drunk at least twice.
Nasa safety chief Bryan O'Connor said he "was unable to verify any case in which an astronaut or spaceflight crew member was impaired on launch day".
Astronauts are banned from drinking alcohol in the 12 hours before flying.
Mr O'Connor's report was based on interviews with some 90 astronauts, flight surgeons and other Nasa officials and covered the past 20 years.
He was also sent an e-mail by 20 flight surgeons vouching that they had never seen any drunken astronauts before a launch or training flight.
Speaking before the launch of the shuttle Endeavour earlier this month, Nasa head Michael Griffin said it was "uncredible" that astronauts could be drunk given the medical tests and scrutiny they underwent before flying.
The independent panel report released in July was commissioned as part of a review of astronauts' health following the arrest of astronaut Lisa Nowak in February.
Nasa was also shaken by the sabotage of a computer bound for the ISS
Ms Nowak is due to stand trial next month on charges of attempted kidnap, assault and burglary after allegedly confronting a woman she believed was a rival for the affections of a space shuttle pilot.
Giving his report, Mr O'Connor said he had found no evidence to substantiate the claims of astronauts being drunk, despite looking back through 40,134 official reports of problems dating back to 1984.
He said he was confident there were "reasonable safeguards in place to prevent an impaired crew member from boarding a spacecraft".
Although there were occasionally disagreements between operations and medical team members, everyone understood the safety concerns and procedures, he said.
Mr O'Connor recommended, however, that flight surgeons should play a stronger "oversight" role on launch days.
The July report, which was based on unverified interviews with astronauts and flight surgeons, cited "some episodes of heavy use of alcohol by astronauts in the immediate pre-flight period, which has led to concerns".
"Two specific instances were described where astronauts had been so intoxicated prior to flight that flight surgeons and-or fellow astronauts raised concerns to local on-scene leadership regarding flight safety," the report said.
"However, the individuals were still permitted to fly."
One of the cases involved preparations for a shuttle mission that was eventually delayed, the panel said.
The second involved a Russian Soyuz mission bound for the International Space Station (ISS).
The July findings were an embarrassment for Nasa, coming soon after the arrest of Ms Nowak and the revelation that a computer supplied by a contractor to be taken to the ISS had been sabotaged.