After taking acetaminophen, it can take slightly longer than 8 hours for it to be undetectable in the bloodstream, according to Magnum-Migraine Awareness Group. This is due to acetaminophen's half-life, which is approximately 1 to 2.5 hours in adults.
A medication's half-life is the time it takes for it to be reduced in the body by half of its original concentration value. Although the half-life for acetaminophen in adults can be up to 2.5 hours and less in children, it is longer for patients with hepatic conditions, such as cirrhosis, notes RxList.
Acetaminophen is an analgesic, which is available under different brand names, such as Tylenol. This is an OTC medication that is used for fevers, arthritic pain and headaches, among other symptoms.