Acetaminophen is usually taken orally and is quickly absorbed from the gastrointestinal tract into the bloodstream, namely the plasma. Its effects are generally felt within 30 to 45 minutes. It is metabolized by the liver, enters the bloodstream and is later excreted via urine. The term “half-life” is used to determine how long it takes a medication or drug to be eliminated from blood plasma by one half of its strength. Once the drug is no longer in blood plasma, it loses its detectability. Therefore, after its half-life ends, the drug is no longer in the bloodstream
The half-life of acetaminophen is 1 to 2 ½ hours in most people, meaning that it is detectable up to that point, but not much longer afterward, as the drug is exiting the blood to be excreted by urine. Within six to eight hours, only tiny, generally undetectable traces of the drug are left.