If it could be one or the other (as in, they know for sure both can cause it), it's probably the combination. Like most antipsychotics can cause it and benztropine can cause it, and taking one or the other is fine for me, but if I take benztropine to treat EPS caused by antipsychotics while still on the antipsychotic, I get urinary retention. If I'm on two antipsychotics, I get it.

It's definitely one of the more frustrating side effects I've ever had, for sure. Especially going to the ER for it and they ask for a urine sample and it's like "YOU IDIOTS I'M HERE BECAUSE I CAN'T ****ING GO!" *cue chucked urine cups and ugly crying followed by bladder scan where it can't even tell them how much liquid's in my bladder because it's more than can be accurately detected* "oh, I'd be in pain too if I had over a liter of urine in me that couldn't come out."

I hope you can get this settled. I'd tell all your docs from now on you're prone to urinary retention and need to avoid meds that cause it.