What are the side effects of an extreme diet?

Wow. Sounds just like me. The side effects in the long term I've heard can be bad skin, being more susceptible to sicknesses, weak fingernail growth, hair falling out. These are extreme side effects though and it hasn't deterred me from doing what works for me - which is apparently not a healthy weight loss strategy but I get the results and that's all I care about. I've had trouble keeping it off so that's what I'm working towards next. As long as you're not completely in starvation mode (I suggest eating 3 VERY SMALL meals a day - example: very small salad or soup). You should be fine. Your body can withstand gruelling diets, its meant to, it may not be the best and most sensible way but do what WORKS for you, I say.