Common side effects of NyQuil include constipation, appetite loss, anxiousness, stomach upset and queasiness, according to Drugs.com. Severe side effects include urinary difficulties, hives and irregular heartbeat. It is important for individuals to seek medical attention if common side effects are persistent or individuals experience any severe side effects.
Additional common side effects caused by NyQuil include sleepiness and thickened lung secretions, explains WebMD. Less common reactions to the medication include hallucinations, vomiting, visual disturbances and excitability.
Ingestion of NyQuil can lead to liver damage if more than four doses are taken within a 24-hour period, according to Physicians' Desk Reference. Liver damage can also occur if NyQuil is taken in conjunction with products that contain acetaminophen. NyQuil should not be taken by individuals who are taking monoamine oxidase inhibitor medications, also referred to as MAOI medications. The drowsiness effects caused by NyQuil may increase if the medication is taken with alcohol or sedative medications. Individuals who are taking tranquilizers or blood-thinning medications should first consult with a pharmacist or physician before taking NyQuil. Individuals with liver disease, chronic cough and chronic bronchitis, as well as those with enlarged prostate glands, should also consult with a physician prior to taking NyQuil.