You will find two ways to get more steroid testosterone into your body. You either stimulate your testicles into producing more, or you take some kind of synthetic hormone to replace what their body could have made. The negative side effects only are derived from using synthetic hormones - boosting your testicle's production have no known side effects whatsoever.
Using artificial drugs will cause your body to stop producing testo-sterone because the brain picks up there exists already too much. In fact the synthetics will stimulate estrogen production to be able to try to balance out the excess of steroids in the body. Taking synthetic steroids will also reduce the testicles simply because they are no extended used to make ejaculation and testosterone. Which of course brings about the other side effect - that being that men helmut strebl steroids on these synthetic drugs almost always are infertile; made clean and sterile by the flood of drugs in their system which close down the testicles doing anything useful? Ultimately, there is a risk of developing prostate cancer as it has been known for a long period that it is testo-sterone that can make a tiny cancer become quite extreme. So, besides breaking the law by taking against the law steroid hormones, there is serious health hazards associated with these drugs.
Contrast this with naturally increasing your own body to make the hormones it needs, there is increased fertility because your testicles are always busy being active doing what they should be doing. There is no disproportionate production of estrogen because the higher level of testosterone is the desired result searched for by the brain, somewhat than the brain trying to control a overflow of foreign drugs. And no risks of prostatic cancers because the increased testosterone is expected and desired by the natural systems within the body. And because the testicles are busy constantly, there is no shrinkage in size.