The combination of dextromethorphan and quinidine is used to treat pseudobulbar affect (PBA; a condition of sudden, frequent outbursts of crying or laughing that can not be controlled) in people with certain conditions such as amyotrophic lateral sclerosis (ALS, Lou Gehrig’s disease; condition in which the nerves that control muscle movement slowly die, causing the muscles to shrink and weaken) or multiple sclerosis (a disease in which the nerves do not function properly and patients may experience weakness, numbness, loss of muscle coordination and problems with vision, speech, and bladder control). Dextromethorphan is in a class of medications called central nervous system agents. The way it works in the brain to treat PBA is not known. Quinidine is in a class of medications called antiarrhythmics. When combined with dextromethorphan, quinidine works by increasing the amount of dextromethorphan in the body.
The combination of dextromethorphan and quinidine comes as a capsule to take by mouth. It is usually taken with or without food once a day for 7 days. After 7 days, it is taken every 12 hours. Do not take more than 2 doses in a 24-hour period. Be sure to allow about 12 hours between each dose. Take dextromethorphan and quinidine at around the same time(s) every day. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take the combination of dextromethorphan and quinidine exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.
Summary from MedlinePlus.gov, click for more information.