There aren't any inherent dangers in dieting if you do it RIGHT.
However, if you engage in crash dieting, there are dangers in that...
Assuming you're referring to crash dieting, that can lead to:
1. Eating disorders
2. Cardiovascular disorders/organ damage
3. Osteoporosis
You asked for 3 but there are more issues..those are just the 3 biggest ones.
Just diet sensibly and don't go below 1,200 calories as a woman or 1,500 as a man..this is a good general guideline to follow.
Make sure you're taking a multivitamin as well.
If you need anymore help setting up a good diet, there's more info here.
