If you've gone through that doorYou know it's not always easy to give up habits even when you know the harm they do. Sometimes, the cost/benefit analysis doesn't look encouraging. In any case, no reason to let the four horsemen loose in your life, just because you happen to like . . . whatever a bit more than you should.
First, if you have murderous habits in your life, be sane about pursuing them. If you use needles, use clean ones and don't share. If you consume narcotic substances, treat the poisons with respect, control the use as much as you can, do all the things you can to minimize ill effects. Notably, eat well and hydrate continually. Water is a universal blessing.
Second, beware the top four killers: Tobacco, poor diet, lack of exercise, substance abuse.