When I was in chemo, I was told to tell my doctors about everything I was taking - over the counter medications, other prescriptions, and any vitamins and supplements. I thought they wanted to know for two reasons - one to have an idea of the bigger picture of our health and to make sure we were taking vitamins. Then today I found this article that says fish oil can block some chemotherapy drugs.
I know there are lots of potential drug interactions out there. But I think they are often thought of between two different prescription drugs as opposed to interactions with OTC medications, vitamins or supplements. I can tell you from personal experience there are often interactions with everything. If you aren't sure, take a look at this list of interactions with St John's Wort.
Between my lack of thyroid, allergy to Benadryl, or more correctly some antihistamine in it, and all my other medications, when I get a cold I ask the pharmacist what I can take. They have to think about this for a bit. No aspirin products because I am on anti-inflammatories - this means no ibuprofen, etc. No antihistamines because of my allergies. Nothing that will interfere with my thyroid medication which has all sorts of interactions. Nothing that has any kind of warning 'do not take if you have thyroid problems'. This means if I have a cough, I can only take the basic Robitussin - none of the ones for all day use or with decongestants too. If I have a headache, either I suck it up (and whine) and take a tylenol. Also, with my thyroid medication that I have to take by itself on an empty stomach one hour before eating or four hours after eating or taking any other medications, this gets complicated.
My husband had a cold recently. I woke up and he was rummaging through the medicine cabinet looking for something. He then went down stairs and moped in front of the TV. When I came down I asked him what was wrong and did he take any cold medicine. The answer was he couldn't find any. Because we don't have any because of my issues. I made a special trip to Walgreens to pick up some heavy duty cold medicines for him. He doesn't have the interactions issues that I do.
When we are traveling I pack our bag of prescriptions and then consider what else to take. Do we need tums? Probably. Do we need tylenol? Maybe. Do we need anything else? I hope not.
And I'm allergic to penicillins and codeine. When I need a prescription, I am a pharmacists nightmare.