I recently found myself in a situation in which an over-the-counter pain medication prescribed by one of my doctors began to poison me, and, given enough time, probably would have done permanent damage to some major organs I've come to depend on over the years. I didn't realize what was happening at the time, I just lost feeling in a section of one hand, as the pains that were supposed to be treated by the drug rose to entirely new levels.
What allowed me to diagnose the problem, and realize that I had to stop taking the pain medication immediately, was a combination of the National Institute of Health's MedlinePlus drug data, and an automatic drug interaction checker that I found about a year ago: Medscape Multi-Drug Interaction Checker. (That's a link to the web version of the checker, but there is also a free iPad/iPhone app. The app has the virtue of remembering the medications you've previously entered, among other things.)
Just enter the name of every drug, supplement, etc. that you're taking, or being told to take, and see what the interaction checker comes-up with. Interactions are classified by their severity, and their effects are described.
If an interaction turns-up, there's a good chance that the description will be very specific, like a reduced ability of the kidneys to eliminate some medication. However, it won't tell you what effects you may experience as a result. In a case like that, logically you know that a build-up of any drug could lead to an overdose situation, but you'll need the NIH's Medline Plus data to learn what effects you might be experiencing as a result. Therefore, neither service is necessarily sufficient by itself, but, together, they can let you ask specific and important questions of your doctors, and maybe spare you some serious nastiness. Of course, when you have to catch a problem yourself, following-up with a good doctor as soon as possible is prudent.
Now, ordinarily, I won't use dot-com sites for health information, because there's no reason to assume they are remotely objective, employ quality controls, keep material up-to-date, etc., but this interaction checker was unique, as far as I could tell, when I found it. Since then, it's done me very good service. So, this is the exception to my rule.
If you know of something even better out there, leave a comment or drop me a line. If you don't, I recommend getting in the habit of using the interaction checker, even if you never take anything other than supplements, over-the-counter meds, caffeine, etc. Forwarned is forearmed, as they say, and since some doctors can't be bothered to gather basic patient histories (like what drugs you're on), to check for drug interactions, or to otherwise demonstrate even rudimentary competence in their fields, you're going to have do that for them, just to protect yourself.
(Old joke: "Did you know that 50% of all doctors graduated in the bottom half of their class?" Unfortunately, the joke's on us patients.)
Disclaimer: I'm a software engineer, not a doctor. I have a better grasp of logic, rudimentary statistics and the basics of the scientific method than some of the doctors I've dealt with over the years, but not one, single medical credential. Don't trust your health to me, please.
Be well, folks.