The premise that medicine gemachs work on is that you borrow medicine that you’re going to give back when you get your prescription filled. It’s not just a free giveaway of medicine.
You can, however, ask what their policy is on giving back because maybe some are more relaxed about that. Get your hands on a Madrich Chareidi and there are tons listed in the middle section under Gemachim. They are listed by neighborhood there.