What do you do with a lie?

I am concerned that my partner has been lying to me and if I find out that he has been, what do I do? 

See we came to an agreement last weekend that he was going to start taking his full dose of his anxiety meds as apposed to the half dose that he had starting taking.  He started to take the half dose over 6 months ago and I have only seen bad things come from that.  He triggers way to easily on only a half pill and every weekend was like a walking on egg shells for me.  He was just acting so nasty towards me on this lower dose.  I actually got to the point of contacting a therapist to help me deal with his PTSD. 

Well after he stepped back and could see how badly he had been treating me he promised me that he would start taking his full dose of meds again.  For a couple days, I think he had been.  But then all of a sudden, he started "hidding" his meds in the drawer when before they sat out in the open on the counter.  He told me that "I was going to have to trust him" that he was taking a full pill still.  Well he started triggering again these past few days.  Plus he kept asking me if I had noticed a difference in him and that it was going to take awhile for the pills to kick in.  I feel like he has just become way to focused on if I am noticing anything different and he is hiding his dose from me. 

So this morning while he was out, I counted the pills in the bottle.  There are 21 whole pills and 9 halfs.  Over the next couple days I am going to count them before he gets home from work.  I absolutely hate not feeling like I can trust him, but I can't.  And what am I going to do if I find out that he has been lying to me???  I mean I just can't say "oh its ok, lets just start taking the right dose" and then just dismiss that he had been lying to me and hiding things from me.  I mean if he can hide that, then what else could he be hiding??  I do know, that if I find out that he has been lying, I will start to see the therapist because I honestly will be at the end of my line.  If he breaks my trust, what else will I be able to do??