Why is Tylenol so sleepy?

Tylenol contains the antitussive dextromethorphan hydrobromide and the antiallergic drug chlorphenamine maleate. Both drugs have central nervous system inhibition and sedative effects, so adverse reactions such as drowsiness will occur after taking Tylenol. After taking Tylenol, it is not advisable to drive, operate machinery or work high above the ground. There are clear requirements in the instructions.