Pharmaceutical compositions comprising bupropion and cysteine

A method of treating agitation associated with Alzheimer's disease in a human being in need thereof with a dosage form comprising dextromethorphan, bupropion, and cysteine, comprising orally administering the dosage form comprising the dextromethorphan, the bupropion, and the cysteine, wherein the dosage form comprises about 45 mg of dextromethorphan hydrobromide, about 105 mg of bupropion hydrochloride, and a cysteine in a salt form, wherein the dosage form has a molar ratio of the bupropion to the cysteine that is about 0.5:1 to about 1.1:1, wherein bupropion and dextromethorphan are the only therapeutic active agents in the dosage form, wherein the dosage form is a bilayer dosage form, and wherein the bupropion and the cysteine are in a first layer and the dextromethorphan is in a second layer.