According to Tylenol, acetaminophen should start working within 30 to 60 minutes. It doesn't work immediately because it first has to dissolve in the digestive tract and then enter the blood stream. Once it reaches the brain, acetaminophen relieves pain by blocking pain signals, according to FamilyDoctor.org.
Acetaminophen is one of the most popular over-the-counter pain relievers. When used as directed, it is very safe. If it is taken in higher dosages, it can cause liver damage, according to WebMD. The drug should also not be taken for longer than 10 days unless the patient is directed to do so by a doctor.