Nyquil and Tylenol are both medicines used to treat cough and cold symptoms. The difference is that Nyquil is used for severe colds and flu and contains the ingredient Tylenol, a brand of medication whose active ingredient is paracetamol. Repeated doses may increase the risk of adverse reactions, so you should wait 4-6 hours before taking a second dose of Tylenol. In older people with a slower metabolism, this is more often every six hours. A single dose should not exceed 1000mg of Tylenol and should not exceed 4000mg of Tylenol in a 24-hour period; taking too much Tylenol can cause liver damage.