I’m confused about caffeine pills.

Vivarin (200 mg), pic related and as of recent drug of choice works way better for me than off brand tablets. Feels like ~8 hours of energy. Then a big crash.

Tried many off-brand caffeine tablets, even double dose. Mostly just headaches though and not the same “clean” alertness.

I assumed quality control or additives or what not. But it’s literally just caffeine, right? Do other people see this? Could tablet fillers / how fast it dissolves matter? Or is this just tolerance + timing + placebo?

What’s the actual mechanism here?

Vivarin is the first time I understand why some people call it poor people’s meth…