There are currently 2 known instances where there is a possibility that laser hair removal can make hair grow back thicker and darker than it was before treatment. Please keep in mind that one is preventable while the other is so rare, the chances of it happening are very small.
The first scenario can happen to anyone, though the majority of people that opt for laser treatment may not be aware of this risk prior to treatment. In order to perform laser hair removal, the technician will require you to shave the desired area a day or two prior to your appointment. The hair needs to be as short as possible, yet still be visible, in order for the laser to work to its full potential. Unfortunately, some areas that people want to have lasered, (such as the face and back), contain a mixture of the dark coarse hairs that they want gone and "peach fuzz" (the extremely fine and colorless hairs that cover a lot of our body). When shaving this area in preparation for laser treatment, the peach fuzz that is present also gets shaved. With shaving, hairs tend to grow back thicker, darker, and faster with every shave. The fine hairs that were once unnoticeable may now start to appear darker upon return, which can create the illusion that more hair is beginning to grow after having laser treatment.
The second instance is a medical condition called "paradoxical hypertrichosis". It is a very rare reaction that can be experienced after laser hair removal treatment. As paradoxical hypertrichosis, (or postlaser hypertrichosis), is still a relatively new and extremely rare condition, there has not been a great deal of research done on the subject. It is difficult to determine what the exact cause of this phenomenon is. Some specialists suggest that this unwanted hair growth can be caused by extremely low intensity laser or IPL treatments at levels that are too low to be effective and actually end up stimulating new hair growth in the hair bulb as opposed to destroying the hair bulb completely. Others suggest that ethnic background may be a factor as research has found that this reaction tends to happen more in people with skin types 3, 4, and 5 (olive and darker skin) than it does in people with lighter skin. There is also the possibility that certain medication can increase the risk of paradoxical hypertrichosis if taken at the same time as the laser treatments. As you can see, sufferers of this condition are so few and far between that it is difficult to get a proper sampling.
Regardless of the suggested factors of paradoxical hypertrichosis, it is extremely important to find a laser technician that is skilled and qualified to perform your treatments. They need to be knowledgeable enough to determine the correct laser intensity setting for your particular hair and skin attributes. Too low can be ineffective and actually cause adverse effects. Intensity that is too high can cause side effects such as minor burns and skin discoloration. The perfect balance will safely and effectively remove your unwanted hair and make your laser hair removal experience a positive one.
Even though it is very rare that laser hair removal will make your hair grow back thicker, it has been known to happen in some cases. If you are concerned, or if you know that the area you want to have covered includes peach fuzz, sometimes a different method like electrolysis hair removal would be a better alternative for you. Electrolysis does not require you to shave prior to treatment so you do not run the risk of shaving over any peach fuzz and causing it to grow back thicker. Each hair is individually treated so you do not run the risk of any other area of your skin being treated other than where you want to be treated.