Anabolic and androgenic steroids are available as prescription medications to be used in cases in which the body does not make enough hormone and supplementation may be required. Some hormone supplements in this pathway include growth hormone and testosterone itself. These medications are legally prescribed by health-care providers, but this group of drugs is often used illegally and abused to help increase athletic performance and improve body appearance. When used in a well-nourished body, anabolic steroids will cause weight gain primarily due to an increase in muscle mass.
Remember, nothing is alleged to--or can have--happened to all of MLB over some one or two seasons: the claim is that PEDs were being used at a slowly but steadily increasing rate (and thus "distorting records") from very roughly 1980 through the present. Were that so, or anything like it, we would expect to see a clear long-term uptrend during this period. But we don't: we see a nearly flat line that, if anything, slopes slightly down. The "boost" just isn't there. But that doesn't seem to stop anyone from talking about it.
One last point: Because steroids have all these side effects, a lot of abusers will then take other drugs to offset the effect—steroid abuse causes you to grow man-boobs, so people take another drug to mitigate those side effects. Guys who take steroids often also take augmenting drugs to decrease estrogen production or to improve athletic performance even further, like human growth hormone, insulin-like growth factor 1, insulin, erythropoietin, stimulants, diuretics, levothyroxine, and/or gamma-hydroxybutyrate, according to the study in FP Essentials . That means on top of all these side effects of steroids, most abusers are also looking at a whole separate list of effects from their peripheral drug habits.