Best product to buy to increase testosterone?

Getting your minerals in check definitely helps. It might have been a placebo effect though, but I tried Tribulus and found it quite stimulating. ZMA (zinc-magnesium) before going to sleep works quite well.

There are also tons of new products for increasing testosterone. I've just found some sort of candy like gummy bears that should help. If you want to learn more, check out the link in my bio.