What did a drug do to the consumer? It gave them a high that one couldn't attain through natural means, making them feel as if they were using all their potential.
Just that feeling of weightlessness was enough to make one addicted. But, once one attained a certain level of power, they could casually do this on their own. So, drugs became meaningless against the powerhouses.
But, what if there was a drug that didn't give them the usual high but made them experience the sensation of growing stronger but at a faster pace?
In a world where strength reigned supreme, it was of course the most sought after. And indeed, the sensation of growing strong was the most desirable.
The Mana Capsule gave exactly that, quenching the unquenchable thirst of everyone with even a bit of power. And for those that failed to improve their strength for years, the effect was even more potent, allowing them to experience the feeling that they had lost.