Why is Tylenol's version of Dayquil listed as being non-drowsy when the first thing it does is knock you out?

Answer 1

Both Dayquil and Tylenol Daytime cold products contain:

Acetaminophen (pain reliever/fever reducer)
Dextromethorphan (cough suppressant)
Phenylephrine (nasal decongestant)

None of these medications typically cause drowsiness. If you are experiencing drowsiness, you are having what is called an idiosyncratic reaction - which means you are sensitive to the product while most other people are not.

Rick the Pharmacist

Answer 2

Certain medications may be labeled as "non-drowsy". This does not mean that no one on earth will react to this drug by becoming sleepy. It means that in a majority of people in a clinical trial reported no drowsiness as a result of the use of this product.

Tylenol, I believe, has a list of possible side effects,ingredients, and contraindications on the side of the bottle. Open the sheet glued to the side of the bottle and look in the second tab. It may contain a drug that you are sensitive to, or possibly alcohol. There is the list of side effects, and one of these (I believe the first) is drowsiness.