Although check the laws of your destination country. A few people are arrested for being in possession of controlled drugs because they took something for the flight - possession by bloodstream.
http://diytravelexpert.com/dubai-danger-medicines-drugs-and-...
(Though that describes Dubai in particular -- I don't know if there are similar concerns elsewhere.)
> Cat Le-Huy was arrested in Dubai for carrying Melatonin jet-lag tablets, which are sold over the counter in the US and Dubai
In that case it seems to make sense to do more research or leave it at home.