In general, certain antidepressants appear to be more likely than others to induce weight gain. Amitriptyline, imipramine (Tofranil), and doxepin are examples of tricyclic antidepressants. These medications may increase the amount of food that you eat or change how your body uses energy. This can lead to weight gain. Paroxetine (Paxil) and sertraline (Zoloft) are examples of selective serotonin reuptake inhibitors. They work by blocking the action of a brain chemical called serotonin, which some scientists believe is responsible for feelings such as happiness and sadness. As a result, less serotonin is available to carry out its role in regulating emotion. Serotonin is also involved with appetite control, so blocking its activity increases hunger and causes weight gain. Lithium affects thyroid function and may therefore play a role in obesity. Lithium reduces the feeling of desire for food and may also reduce the ability of the body to use fat for energy. People who take lithium should not change their diet without first talking to their doctor. Eating more calories than you burn will only make matters worse. Anticonvulsants are drugs used to treat epilepsy and other disorders associated with seizures. They work by reducing the frequency of seizures and sometimes by preventing them altogether.