A bit over decade ago when I started running (& cycling) hardcore I had some knee pain and started taking Glucosamine & Chondroitin supplements for it. It seemed to work, maybe it was placebo effect, but regardless I was "sold" on it. A year or so later I started getting hives all over my body. I was (and am) quite healthy, and coupled with the fact that I'm not really allergic to anything this took me by surprise. It got bad enough that I had allergy panels done, which unfortunately came back with allergies to all sorts of things and the docs put me on anti-allergy medication (Zyrtec, along with stronger Rx only stuff). I was quite discouraged, esp when the docs told me that one can become allergic to anything at any point in one's life, and I was slowly starting to accept it. The Zyrtec seemed to help, not 100%, and I didn't seem to have to take it regularly, so it wasn't the end of the world. Then, at one point I took a 10 day trip and didn't take my supplements with me. The hives went away. Thru the process of elimination I traced it down to the Glucosamine & Chondroitin supplement. It's probably been a decade since I've been back to my normal, non-allergic self. I still take Glucosamine & Chondroitin supplements for a period of time at a time, but am keenly aware of not letting things build up to toxic levels after this lesson learned.