Some patients have a paradoxical stimulant effect from it, with rapid heart beat and some trouble falling asleep. If these side effects are severe, other medications can be added to block them. Amitriptyline may also cause a craving for sweets and weight gain. I recommend you avoid sweets entirely while on amitriptyline. Exercise will also help to keep your weight down, as will eating a low-fat diet such as the Weight Watchers one.
Some patients report that such a diet also helps their fibromyalgia symptoms.
Constipation is another frequent side effect. Magnesium supplements are often helpful for constipation and may also help fibromyalgia symptoms in some patients. Several other medications have been used to treat fibromyalgia, although none seems to work quite as often as amitriptyline. None of these other medications cause weight gain the way amitriptyline does. While I can't recommend them simply because they haven't been adequately studied for efficacy or long term harm, I don't discourage patients from using them if they find them helpful.