I'm not 100% sure as to when/why/how. I've finally gotten the pills to drop. Since they started dropping for me since I've updated my game to latest version (1.3.17). But either it requires somewhere around 1000% drop rate. Or beating the games current final stage. I cannot tell you what actually causes it as I basically achieved both of these. Within the same 10-20 minutes of each other.
To be utterly precise. I mean just 'being able to use' 1000%. Not actually using it via difficulty spike. I'm leaning more towards the idea that it's based on completion. Of the last stage to be precise, again. Because I sure haven't completed the last stage above difficulty lvl 21. And they still are dropping them with surprising ease.