The reason why Dayquil is age restricted is due to safety concerns as well as potential for misuse, possible drug interactions and even due to possible production of methamphetamine.
Children that are under the age of 4 years old are the most vulnerable to the side effects of the ingredients in Dayquil.
Dayquil contains ingredients like pseudoephedrine and dextromethorphan which can cause serious health issues, especially in children under 4 years of age.
The serious side effects of the pseudoephedrine and dextromethorphan include respiratory depression, hallucinations and seizures.
So children under 4 should not take Dayquil.
The dextromethorphan in Dayquil also is an hallucinogenic drug, which can be abused by people, especially by teenagers and young adults.
And so limiting the access of the Dayquil, helps to prevent possible misuse and reduce the risk of intoxication.
Dayquil also can interact with other prescription medications, like antidepressants and sedatives.
And so limiting the use of Dayquil to adults can also help avoid and prevent potential drug interactions, which could be dangerous.
And the pseudoephedrine ingredient in Dayquil is also a key ingredient in the production of the illegal and dangerous drug methamphetamine.
And so by restricting the sale of Dayquil to adults also helps reduce the risk of it's diversion and use for illegal purposes.