- Marijuana: really bad for spermatogenesis for a multitude of reasons. Also bad for sperm themselves
- Cigarettes: see marijuana
- Abdominal obesity: surprisingly bad. Causes both hormonal imbalances and sperm stem cell death
- Testosterone injections: causes decrease on the natural T production needed to produce sperm
- Heat: the testes hang in the sack for a reason. Keep them cool by avoiding hot tubs, spas and steam rooms
Thanks