Those using NyQuil should strictly follow the dosage chart or the physician's prescription. Overdose by increasing the amount of intake, taking it more often or extending the intake can cause serious side effects, such as brain damage and seizure, according to WebMD.
NyQuil is a medication that has the active ingredients dextromethorphan, acetaminophen and antihistamines, notes WebMD. It is used to treat the symptoms of allergies, the common cold, flu and other breathing problems. The recommended dosage is based on the patient's age, weight and medical condition. The product is not recommended for children below 6 years old, unless the doctor prescribes it. NyQuil should not be taken together with medications that have the same active ingredients.