Side effects with carbidopa are rare and usually occur due to the levodopa-containing product that is taken together with it. The following side effects in this section may occur due to levodopa. Dizziness, lightheadedness, nausea, vomiting, loss of appetite, trouble sleeping, unusual dreams, or headache may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. This medication (or levodopa) may cause saliva, urine, or sweat to turn a dark color. This effect is harmless, but your clothes may be stained. To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Some people taking this medication along with levodopa/carbidopa 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 carbidopa and levodopa/carbidopa even if you have used these medications for a long time. If you experience increased sleepiness or fall asleep during the day, do not drive or do 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. Tell your doctor right away if you have any serious side effects, including: new/worsening movements you can't control/spasms, greatly increased eye blinking/twitching, vision changes (such as blurred/double vision), eye pain, severe stomach/abdominal pain, black/tarry stools, vomit that looks like coffee grounds, mental/mood changes (such as agitation, hallucinations, depression, thoughts of suicide), signs of infection (such as sore throat that doesn't go away), easy bleeding/bruising, unusual tiredness, tingling of the hands/feet, unusual strong urges (such as increased gambling, increased sexual urges). Get medical help right away if you have any very serious side effects, including: chest pain, fainting. Abruptly stopping or reducing the dose of this medication and levodopa/carbidopa may rarely cause a serious condition called neuroleptic malignant syndrome (NMS). Get medical help right away if you have any of the following symptoms: fever, unusual muscle stiffness, severe confusion, sweating, fast/irregular heartbeat. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Some products that may interact with this drug (or levodopa) include: certain drugs used to treat high blood pressure (such as methyldopa), tetrabenazine. Taking MAO inhibitors with this medication (along with a levodopa-containing product) may cause a serious (possibly fatal) drug interaction. Avoid taking MAO inhibitors (isocarboxazid, linezolid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, tranylcypromine) during treatment with this medication. Most MAO inhibitors should also not be taken for two weeks before treatment with this medication. However, certain MAO inhibitors (rasagiline, safinamide, selegiline) may be used with careful monitoring by your doctor. Ask your doctor when to start or stop taking this medication. This medication (or levodopa) may interfere with certain lab tests (such as urine catecholamine/glucose/ketone tests), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.