Only once did I have a strongly negative reaction to a supplement. That was the herbal extract ephedra which contains ephedrine, a powerful stimulant that greatly benefits weight loss but with some very negative side effects. I figured out it was not for me ... long before FDA banned it. I cannot think of any other supplement in the last two decades which has been implicated in seriously negative side effects. Supplements do not have a record of serious side effects like pharmaceuticals which have even resulted in death in rare cases. Nonetheless, sensitive people may find they have problems with mega doses of some supplements. Alpha lipoic acid is an example of an excellent supplement for diabetics that I need to be careful with. I used to take 3 x 600mg a day which I regard as the maximum reasonable dose. ALA is very acidic and I found that large 600mg capsules sometimes gave me a little heartburn. Now, I take 4 x 300mg/day with no problems.
I do take many supplements. I do believe that therapeutic doses are often mega doses. I do not think that anybody should add more than one new supplement at a time and it is best to increase dosage gradually.
Will supplements dramatically improve your health? I do hear accounts of significant improvement related to supplements, especially fish oil and Vitamin D3, but most supplements are an act of faith. I take my supplements regularly and I notice nothing. It seems as if nothing has happened -- nothing at all. No heart disease, no neuropathy, no retinopathy, no kidney disease -- nothing at all.
And that's why I take supplements.