Not all diets are dangerous. Just make sure you're getting all the nutrition you need (i.e. grains, fruits, vegetables, protein, etc.) Don't do any diets where you only eat like one or two different foods for a while, don't diet with laxatives, make sure you stay hydrated, and don't starve yourself.
The best diet is just making sure you're getting more healthy foods rather than just cutting back on portion sizes.
Well, if you overdose on diet supplements it can be deadly. Also, if you dont need them, you can get addicted and maybe become bulemic or anorexic.