The lasers used in laser therapy for hair loss are low level lasers. The use of "cool" laser light in laser hair therapy is debated by physicians, with some dismissing it and others embracing it. The US Food and Drug Administration has approved the use of laser hair products such as the HairMax LaserComb, but whether the FDA approval indicates that the FDA considers the tool effective, or whether it merely considers it safe is not clear. The company that makes HairMax released documents to the FDA, but has chosen not to release those documents to the public.
The use of cool lasers in hair loss treatments dates back to the 1960s when a Hungarian researcher exposed shaved mice to cool laser light similar to UV light emitted by the sun and discovered that their shaved hair grew back at a much faster rate than the shaved mice in the control group, who were only exposed to regular light.
Laser hair treatment involves applying laser rays directly to the scalp, a process that stimulates red blood cells, releases energy, and causes changes in cellular metabolism. With additional nutrients and oxygen being provided to the scalp, overall scalp blood circulation improves. There have also been claims made as to an increase in the diameter of the individual hair shafts.
The laser hair growth treatment device used by some hair loss doctors looks like a hood hair dryer. It is placed over the top of the head, and the low level laser rotates, causing red laser rays to contact the scalp. But promoting growth of healthy hair isn't necessarily the same as increasing growth of the miniature hairs that result from male pattern baldness or female baldness. Physicians differ in their opinions about whether laser treatment is an effective hair loss treatment. 

While some doubt its effectiveness, others believe that it can increase hair shaft diameter in some patients with male pattern hair loss, and some physicians believe that laser hair restoration results in faster transplanted hair regrowth and quicker resolution of "shock loss" that occurs after hair transplantation.

The biggest risk when it comes to laser hair restoration is cost. The laser itself is not known to have any ill effects or side effects because it is a a low level, cool laser. But paying for the laser tools in a doctor's office can be expensive, and clients are charged accordingly. Laser hair brushes like the HairMax LaserComb cost around $500. While none of these treatments is likely to cause any problems, they are not uniformly effective in all people with hair loss.