Laser based therapy is an excellent way to treat unsightly birthmarks. The usual approach is to undergo multiple treatment sessions spaced out by a few weeks to gently and gradually reduce the pigmentation in the birth mark until it approaches the color of the surrounding skin. Not all types of birth marks respond equally well to laser based therapy, but most people will experience at least some resolution after completing the treatment course.
The side effects of laser therapy are almost always relatively benign. However, in some cases hypopigmentation or hyperpigmentation (having skin color lighter or darker than the surrounding skin) can occur. Obviously, the dermatologist
who will be performing the procedure is an expert in minimizing the chance of this side effect, by making sure that the multiple sessions are completed with the lowest intensity laser exposure necessary each time (the hypo or hyperpigmentation tends to come from an inflammatory reaction that results from "overdamaging" the skin with each treatment).
Setting up a visit with your dermatologist before you have the procedure done is the best first step, as they will be able to answer all of your questions and discuss the risks and the benefits of the procedure in greater detail.