When you take a couple of Tylenol tablets, the active ingredient (acetaminophen) is absorbed into your system and then gradually dissipates, decaying exponentially. Though individual responses may vary, typically acetaminophen has a half-life of around 180 minutes. A single regular strength tablet of Tylenol contains 300 mg of acetaminophen. From clinical trials it is known that the minimum amount of acetaminophen needed in your system to receive any therapeutic effect is 200 mg. About how long after you take two regular strength tablets does the therapeutic effect wear off