Well they don't normally check Amatuer boxers (at entry level anyway) but if its Anabolic there are likely to be side effects, short and long term and the performance increases you get can go aw quickly as they came.

You don't need it to gain weight, Stick to a high calorie diet, full of protien and carbs work hard and let your body rest. You don't need to take that stuff for boxing and even when you know what your doing it can be touchy stuff, it won't just affect your fitness but also your health.

Stuff like Creatine etc isn't bad but steer clear of anabolics.