Bipolar Disorder Support Group
Bipolar disorder is not just a single disorder, but a category of mood disorders marked by periods of abnormally high energy and euphoria, often accompanied by bouts of clinical depression. This is the place to talk about your experience with bipolar disorder, learn from others' experiences, and find support.

Not sure if it is the benadryl
So why does he not understand that bipolar is an actual illness in the brain? You take meds, and they make you feel better. Craziness is not bipolar disorder.
Besides even normal people lose things and forgot where they put it. It has. None thing to do with bipolar, unless you are so heavily medicated you are in a brain fog. If it is brain fog, talk to your pdoc. I was over medicated for 5. Years. I stopped taking prednisone for another disease, and got off my drugs, and was able to live quite normally, well, except for one thing! But better than the 5-6 drug combo my pdoc had me on. He even apologized for over medicating me.
Perhaps in the future instead of jumping to blame someone, you might first look for the item, and if you don't find it, then ask someone for help instead of accusing them of taking it. We do have a Mental Illness, and it can make us forget where we might placed something....but it doesn't give us a free pass to treat our loved ones badly or accuse them automatically.
Working with them and asking for help will probably be much more of a positive less stressful experience in the future.