Marketed by Procter & Gamble, the products Nyquil and Dayquil are available over-the-counter for cough and cold relief. Nyquil is the liquid suspension intended for nighttime use, whereas its counterpart, Dayquil, has a slightly different chemical structure, which does not induce drowsiness, making it safe for use throughout the day. Both medications contain the active components of acetaminophen and dextromethorphan, which are responsible for the majority of side effects associated with these drugs.
Responsible for suppressing cough, one active medication in Dayquil and Nyquil is dextromethorphan. According to Drugs.com, side effects of this medication include nervousness, tremors, drowsiness and dizziness Additionally, 30 percent of those who take dextromethorphan report sleeplessness. Headaches and a feeling of anxiety are also reflective of nervous system side effects. The central nervous system side effects solicit the warning not to drive while taking these medications.
Side effects of both dextromethorphan and acetaminophen impact the gastrointestinal system. Both medications contain irritative properties that can upset the stomach and intestinal lining. Lippincott, Williams & Wilkins define these side effects to include nausea, with or without vomiting, and stomach pain. Acetaminophen can induce nausea and vomiting, as well, by provoking a side effect in the liver.
Acetaminophen is the first active ingredient listed on both Nyquil and Dayquil labels, indicating that it is the most prevalent ingredient in the suspensions. Side effects pertaining to liver health from this ingredient include jaundice, or a yellowing of the skin and eyes. According to the United States Food and Drug Administration, 9.7 billion doses of Nyquil were sold to consumers who may not be aware they were ingesting an acetaminophen-containing drug.