Nausea, vomiting, diarrhea, unwanted/uncontrolled movements, increased sweating, drowsiness, tiredness, dry mouth, gas, and abdominal pain may occur. If any of these effects persist or worsen, tell your doctor promptly.
There is some serious side effects to this medication, and if you are encountering any of them, notify your doctor right away. These side effects are listed below; mental/mood changes (such as confusion, hallucinations), unusual strong urges (such as increased gambling, increased sexual urges), fever, muscle stiffness, persistent diarrhea, easy bleeding/bruising, sore/painful muscles, muscle weakness, trouble breathing, pain with breathing.
A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.
Some people taking entacapone have fallen asleep suddenly during their usual daily activities (such as talking on the phone, driving). In some cases, sleep occurred without any feelings of drowsiness beforehand. This sleep effect may occur anytime during treatment with entacapone even if you have used this medication for a long time. If you experience increased sleepiness or fall asleep during the day, do not drive or take part in other possibly dangerous activities until you have discussed this effect with your doctor. Your risk of this sleep effect is increased by using alcohol or other medications that can make you drowsy. See also Precautions section.