(these are my subjective experiences) Examples: taken large doses of iron (with natural vitamin c from orange or lemon juice) makes me very jumpy to the point of becoming anxious, it also upsets my bowels alot (which is a known side effect). Most ginseng and guarana supplements I have tried give me a lot of enery but also make me very moody (perhaps because of too much energy). Also most multivitamin supplements make me nauseous. Also for some supplements (for example ginko biloba) you should take caution if you also take some sort of medication (even asperin) for they might produce nasty side effects when taken in combo.