According to Vicks, the recommended dose of Nyquil for adults and children 12 years or older is 30 milligrams every six hours. Exceeding four doses in 24 hours can cause severe liver damage. If overdose occurs, it is important to consult a medical professional as soon as possible.
Drugs.com explains that Nyquil should not be ingested in any amount by individuals with high blood pressure, heart blood vessel disease or heart problems. Individuals who consume more than three alcoholic beverages per day or have a history of adrenal gland problems should consult with a physician prior to taking Nyquil. Medications such as beta blockers and MAO inhibitors can increase the side effects of Nyquil.