The maker of NyQuil, Vicks, does not recommend using any of its products after the specified expiration date on the product's packaging. Vicks recommends that expired products are properly disposed of, and Vicks suggests that consumers noticing expired products on store shelves should bring the product to store personnel attention right away. In addition, Vicks asks that consumers always check the expiration date on its products before purchase.
The United States Food and Drug Administration cautions consumers that using expired medicine can be harmful to human health. The expiration date on NyQuil and other medicines is used to make a determination as to whether the product is safe to use and if it will work as it is formulated to do. When a product expires, it is less effective because of changes that occur to its chemical composition and its potency.
Medicine stored in the humid environment of a bathroom medicine cabinet can cause a decrease in the potency and effectiveness of a particular product. The FDA recommends storing medicine in a controlled environment to ensure the proper shelf life of the product. According to the FDA, it has required the use of expiration dates on over the counter and prescription drugs since the late 1970s.