OTC anti-inflammatories have zero effect on me (if I get a headache, the only thing I can do is take a nap). Of course, caffeine and sugar have no effect on me, either, except that they upset my stomach a bit if I ingest them when I haven't eaten in hours. The only drugs I seem to be sensitive to are some antihistamines, but not all of them, and my allergies aren't that bad any more.
I'm lucky in that I rarely get sick and have never been seriously ill (just the routine flus and bronchitises). I had really bad acne when I was younger and have taken pretty much every antibiotic known to man, and did fine as long as I took them with food. I took Acutane for a long time but the only side effect I ever got was the reduction in night vision, which reversed when I went off it--I didn't have any liver problems and my cholesterol only went up a tiny bit. So, I guess I don't usually suffer side effects, but I'm not sure how well the drugs help me, either (I'm 25 and I'd still like to trade my skin in and start over).
My mother, on the other hand, has 2% kidney function and has been on dialysis for 2 1/2 years. 2% kidney function isn't something you can compensate for conpletely with diet. She's on the list for a transplant and I know we'll all be thanking out lucky stars for prescripion anti-rejection drugs if she gets a new kidney.