So, some backstory... we both have ADHD, and are on different but similar medications (him vyvanse, me dexedrine). I often don't take my afternoon dose and skip some days so I have told him that I don't mind if he takes one of my pills occasionally. I knew that he occasionally helped himself to a little more than that, but it never interfered with my dosage and it was fine with me.

Fast forward to two days ago. I go to take my morning dose, and realize how light the pill bottle was. I counted out my pills and only had 13 left. For perspective I am dispensed 90 pills/month, and I refilled my prescription August 30th. That means that in 11 days he took half of my monthly prescription, assuming I took three pills a day (which I am sure I did not).

I confronted my partner and he sheepishly admitted he had been taking 4 of my pills a day (more than my prescribed dose of 3) because he had been doubling his dose of his medication and wasn't due to renew his prescription yet. And per my math he's being conservative about the amount. He promised not to do it again, but I'm still kind of pissed. Now I don't have enough medication to last me the month, even if I halve my dose. But I'm not just pissed- I am worried. This is creeping startlingly close to abuse.

One issue I know is that my medication is more helpful to him than the one he is prescribed, but for whatever reason his doctor won't give it to him. But other than that I don't think he has any valid excuses. I'm worried he's taking the medication for the high (you are supposed to reach a therapeutic tolerance to prescription amphetamines and not feel 'stimulated' by them).

How can I a) let my partner know how concerned I am and b) make sure he stays away from my meds?

TL;DR: My partner helped himself to my prescription to the point of detriment to me, I'm worried he is abusing it and pissed that he was so selfish.