*Disclaimer: This post was written by a total non-expert based solely on internet searches.
Acetaminophen, the main ingredient in modern Tylenol, was first synthesized in 1878, and Joseph von Mering first discovered its analgesic effects in 1893.
Since its launch in 1955, Tylenol has been consumed in massive quantities while its safety was verified, but exactly *how* it kills pain remained unknown. It was confirmed to interact with COX like NSAIDs (Aspirin, etc.), but it barely caused the typical NSAID side effects like reduced inflammation, stomach issues, or blood clot inhibition.
So for a long-ass time, scientists couldn't give a clear answer as to why this motherfucker actually works.
After John Vane won a Nobel Prize in 1971 for proving Aspirin works through COX, everyone assumed Tylenol was similar. Major hypotheses included: 1. It inhibits COX-3 (not 1 or 2). 2. It boosts serotonin pathways in the brain. 3. It acts on the POX site of COX to kill pain in the brain without affecting body inflammation. But none of these fully explained the mechanism.
HOLY FUCK FINALLY!!!!! In June 2025, the mechanism behind Tylenol's pain relief was discovered!!!!!!!!
As Acetaminophen breaks down in the body, it creates a substance called AM404 (N-(4-hydroxyphenyl)arachidonoylamide), which blocks pain signals from traveling up the nervous system (TRPV1) before they reach the brain. This solves one of the biggest questions: 'Isn't Tylenol's effect too fast for it to be acting solely on the brain?'
Our bodies have natural pain-killers called cannabinoids (similar to weed). It turns out AM404 prevents the reuptake of these cannabinoids, bumping up their concentration to kill pain.
Previously, there was no clear explanation for why combos like Tramadol (an opioid-ish prescription drug) and Acetaminophen work so much better together. With the discovery of AM404, we now know why Tylenol and Tramadol are such a power couple.
TL;DR: 1. We used Tylenol for 132 years without knowing why it worked. 2. We assumed it was COX because of Aspirin. 3. Turns out the COX thing was a total bluff; the real mechanism was just found. Newly discovered mechanism: 1. 'Does it even touch COX enough to matter?' <- It actually didn't matter much. 2. it blocks pain channels throughout the nervous system. 3. It activates natural weed-like substances (cannabinoids) in the brain to suppress pain.
"Users are shocked that such a common drug was a mystery for a century, leading to theories about 'Tylenol fairies' and memes about it being a legal drug."
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