Good question. But there is no way to prove that I would’ve been worse off had I not taken the many vitamins I take.
I was taking a supplement for high cholesterol, Red Yeast Rice, that supposedly lowered cholesterol. I started it right after having my cholesterol tested and took it for 6 months and had it tested again. There was no difference in my cholesterol. It did not lower it. I quit taking it.
If there were a way to test all of them, I would say they probably do not work. But my grandmother, who died at 98 swore by them, so I take them.

Late response but one time I had weird red spots which was schamberg’s disease diagnoses by the dermatologist. Spent a lot of money on expensive creams and it did nothing. Did online research and saw rutin and vitamin c regularly helped. Supposedly there was no cure but it disappeared after a few weeks of supplements.

I got HPV a few years ago, the kind that may cause cervical cancer. It was persistent and I had surgery but it didn’t help. My doctor recommended supplements like a multivitamin to see what happens. Several months later at the check up I was cleared. Not only that but my PVCs disappeared. I had akippwd heartbeats for several years anywhere from 10–20 per minute 24/7 which was terrorizing.