Side effects that may occur while taking this medication include: Upset stomach, Anxiety, Nervousness, Muscle or joint pain, Restlessness, Irritability, Feeling lightheaded, Fainting, Increased thirst, Weakness or tiredness, Insomnia, Decreased sex drive, Impotence, Difficulty having an orgasm In case of any of these serious side effects, contact your doctor immediately: Blurred vision or eye problems, Fast heartbeat, Mood changes, Hallucinations, Confusion, Nausea, Low fever, Loss of appetite, Dark urine, Clay-colored stools, Jaundice, Ear pain, Skin rash or itching, Wheezing, Difficulty breathing