Does anyone have a good source for information on the interactions between supplements (vitamins, etc.) and prescription medications?

I would talk to your pharmacist. They are more knowledgeable about interactions between Medications.

Thank you to those who answered! I had some blood work done, generally good, but was told by an internist whom I like & trust that I could do with Prescription Strength D. (I live in a part of the country where the sun can be a bit on the rare side). The way it works is that you start out with the prescription and then go to an OCT dose. I do more walking than anyone else in the senior place where I live with some 40 residents. It's not really by choice but that's a long story... I have had some yet-to-be resolved "issues" with a thyroid dosage, and both my PCP and endocrinologist said for some of the blood tests to go off the supplements first (biotin, calcium, C, + magnesium) but that didn't seem to matter. I'm not fanatic about supplements but do take them on a somewhat irregular basis.

Call the pharmacist. This is a question for a professional. Or, call your LO's doctor. This is too important a question to ask untrained, anonymous people with no accountability.

It is RealyReal who has some knowledge about supplements, in my experience, Christine.
This is what I would personally do. I would ask my pharmacist.
I don't really believe in any supplements other than d-mannose. So I am not up on them at all.
If you are adding a supplement--let us say D-Mannose, then go to your favorite search engine and type in, for instance:
D-Mannose side effects
D-Mannose drug interactions
D-Mannose and tylenol
D-Mannose and tenormin
D-mannose and insulin
And so on. You can do a lot of research yourself.
You can ask the pharmacist.
Too much worrying about all this and I am betting a good brisk walk would do you more good than all the supplements in the world, hee hee!