Information from the American Lung Association lists potential side effects as
Nicoderm, Habitrol, Nicotrol, and Prostep all manufacture forms of the patch, which works by continuously passing nicotine (in low doses) through the skin and into the bloodstream. Supposedly this helps the body fight cravings for nicotine that are present after quitting, which is the symptom (ranging from grouchiness to severe headaches) that usually causes would-be quitters to return to their old habits.
According to the Nicoderm CQ website the patch consists of a 10 week plan with 3 different levels of nicotine patches. The catch is that you must stop smoking before beginning the program, or else your blood will get too much nicotine in it.
The first 6 weeks use the 21mg patch, then for the next 2 weeks it decreases to 14mg, and the final 2 weeks have a 7mg patch. This simulates a gradual weaning from the nicotine, hopefully producing fewer side effects. The instructions are very precise, and the site indicates that you must follow the instructions carefully in order for it to work.
It is very clear that the program only works once the person has made a personal commitment to stop smoking as well – the patch cannot override the mind.