I suspect you'll find that those prescription drug services do not allow forwarding of mailed drugs. You would have to change the mailing address with the service instead of using PO forwarding. I'll bet a lot of financial things are the same way.
If you know you are going to be spending time away from your regular address, it is usually best if you change your address with each of the mailers you want to send you stuff. I used to work for the Post Office, and employees there were always instructed to do their best to get you your stuff, BUT there are rules that must be followed, too. And if somebody sends you something with "do not forward" on the envelope, or it is a class of mail that is not forwarded, then they won't do so.