A lot of coaches try to persuade against mixing lifting weights with boxing. I think they are just old fashion and/or ignorant. I lifted weights on and off throughout my career and could tell when I was at my best was when I lifted weights. I always felt strong when I was lifting and my handspeed was at least the same. Always do stretches and read up on it to customize your workouts for boxing. Weightlifting is now a part of a high percentage of boxers. You would do yourself justice if you got with the program. Do not let your opponent have an advantage. I actually was lifting quite hard during my training before my last Golden Gloves and the weekend before I won the state bench press championships and broke the record. I won Golden Gloves also. For awhile I was the only weightlifter in my gym but now most of us lift weights. Constantly gauge your weight and I wouldn't recommend taking anything more than a multivitamin and maybe a light protein shake in place of meals if you are cutting weight. Leave creatine and the other mass making supplements alone.