One of the common questions asked by the general public is whether you can maintain your body’s muscular shape and physical gains once weaned off steroids. The answer is it is not scientifically proven as it depends on each individual’s genetics.
It is not compulsory to continue the steroids cycle if you are satisfied with your body shape, however, your body will tend to lose all extra muscles gained from steroid use due to the breakdown inside the body. Therefore, all the hard work of exercising and dieting will fail in retaining the muscles.
source: http://forums.sherdog.com/threads/if-brock-lensar-came-back-usada-clean.3563017/page-3 (This is Chris Cormier before and after. However this is not proven that anabolic steroids will affect reducing muscle mass)
The reason why the body loses extra muscles is because after ceasing anabolic steroid use, a normal body’s natural testosterone levels will shut down due to steroid usage. Hence, without any forced testosterone and lack of natural testosterone production within the body, muscles will begin to break down rapidly. This will result in the continuous loss of your steroid gained muscles, leaving behind just a base body.
source: http://supplementyouneed.com/7-foods-that-lower-testosterone/
However, to prevent losing your muscles, you can try Post Cycle Therapy (PCT) to slowly regain your natural testosterone production. One piece of advice would be to not splurge on natural testosterone booster supplements as most of them are useless.
Steroids In The Fitness Industry 