What product was it in the US and how frequent was it used?
I do not know. The episode I know of is from the memoir of a 101st soldier who was told to take a pill before boarding the plane for D-Day--he said he didn't know what it was, but did as he was told. I believe there's been studies on it, as well as the German use before. It doesn't seem very surprising to me. Drug use and war go hand in hand throughout history.
I have always understood that the pills issued to airborne before Normandy were motion sickness (http://ww2awartobewon.com/wwii-collectibles/motion-sickness-pills/) and so tended to put the men to sleep rather than pump them up. That said, I believe that there was a great deal of drug use by both sides.
I can imagine as an emergency drug in some conditions it would still be stocked, maybe some of the ADD meds, too.
From what I have seen online it looks like the allies supplied amphetamines to service men: https://www.reddit.com/r/AskHistorians/comments/1omm8m/what_role_did_amphetamines_have_in_wwii_by_both/ https://www.ncbi.nlm.nih.gov/pubmed/22073434. I can't tell you the extent. It seems like German use was more extensive, but that is only my impression.Well, the caffeine and nicotine the US pushed onto the armed forces probably made up for a lack of crystal meth
Well, now we know where the inspiration for the 'Nazi Zombie ' video games came from.
On-line searches produce inconsistent reports on Japanese military usage of amphetamines, with some saying that it was common and others suggesting that it was rare or unusual. That inclines me to believe that any such usage was informal rather than systematic.
