What is missed, as
@Johnrankins pointed out, is the scale involved.
Ten million sound like a LOT of meth, mostly because it is, but when you are sending it to a couple million troops over a four month period, it really isn't. Even of you shrink the number down to 500,000 troops, that is still only five pills a month. Most troops probably took it one in a while if they had guard duty (same reason a lot of guys use Copenhagen these days, extra little jolt can make all the difference) or were falling asleep behind the wheel.
Most usage was likely by drivers, tankers, and pilots. The U.S. used to issue Bennies to pilots for use "as needed" in capsule form. According to Bob Johnson, the pilots were told that if they needed a REAL jolt of energy they should open the capsule and dump the contents into their mouths (he describes it in some detail on hos classic book
Thunderbolt).
The Heer fought too hard and too well to be full of tweakers.