You will need to have a sit down with him that the side effects are far more dangerous than what it is worth. Not sure how serious the MMA group he is in takes steroid use in respect to the illegal side of them but if they do take it seriously and could kick someone out who uses them you may want to remind your bf of that.
The side effects though if you want a quick ballpark of the things that can happen: baldness, low sperm count, decrease in testicle size, severe headaches, increased risk of strokes and blood clots, enlarged prostate, development of breasts, liver damage (can be quite severe quite quickly), mood swings as well as increased aggressive behavior (you have already seen this happen), bowel and urinary problems, increased risk of tendon injury, impotence, aching joints, bloating, nausea, changes in cholesterol, high blood pressure and increased risk of heart disease and cardiac arrhythmias, stunting of growth, and severe acne (it can get to the point of extreme scarring and splitting of the scars into open wounds). If you would like graphic depiction of things such as the acne which does happen quite often simply Google "side effects of steroid use". The scarring is deep, dark, and very pronounced and next to impossible to fully get rid of.
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