Soy has got a lot of bad press over the years. It has been linked to lowering testosterone in men and increasing phytoestrogens. This has only been noted in isolated cases where the intake of soy protein was extremely high and the diet itself was unbalanced.
The majority of the research concludes that the risks of soy as a “feminising” food has been overstated. Soy will have a largely neutral effect on testosterone if combined with a balanced diet.
For most people, soy protein is considered safe with little to no side effects as long as you are not allergic to soy.