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The valium should be taken one hour before the PET scan, the effect will start working before this however the one hour allows time for blood levels to rise.
The length of time of the effect of valium will vary for different people, I would expect a good effect for 5-6 hours, and certainly should not need repeated until 8 hours after the first tablet.
It will make you drowsy and affect concentration so do not drive after taking it for at least 12 hours.