I have learned to ask people to differentiate by saying, ok so does it make you itch, get a rash, or does your throat swell up and give you trouble breathing? If yes, allergy and that's that. If the answer is no, say ok what do you not like about this drug? Sometimes they dislike the effect for a good reason, example I dislike taking muscle relaxers with my blood pressure meds because well, they're too strong in combination. Or, they don't seem to work. Sometimes the patient dislikes an effect that turns out to be an atypical adverse reaction, and I put it down as allergy with explanation, because the pharmacy doesn't allow us room in the computer for "atypical adverse reaction," only allergy.
I'm sorry the response isn't cut and dried and easy, but people aren't easy, they are pretty complex sometimes.
Edit: I have to tell you the funniest allergy I have EVER seen. The allergy tape on the chart read: (long list of drugs, blah blah blah, THE COLOR PURPLE.) We were all like WHAT?! Do they mean purple nitrile gloves?? Then we read the History and Physical, and it's there in print, patient is allergic to blah blah blah and the color purple. We asked the doc in the morning and she was quite annoyed, crossed that out and initialed it, and said some people were talking too loud while she was dictating and no, the patient was not allergic to purple.