Stimulants for fitness.

[Petertherabbit]

Stimulants ie amphetamines were a part of aircrew emergency ration kits during WW2.
The thinking was, to be safely rescued after ditching etc, there was a time limited window of opportunity, in which to make it to realistic safety or rescue.
Stimulants, give or gave the aircrew, the energy they needed to make it out of harms way, back into safety, before capture or exhaustion set in.

I'm not sure if they're still used, I would imagine they are and almost certainly in the case of special forces.