How Did We Get It Wrong? Paracetamol Relieves Pain in a Way That's Completely Different Than We Thought

A new study has revealed that paracetamol (or Tylenol) may be able to relieve pain in a different way than previously thought. Instead of increasing feel-good chemicals, it appears to block the production of one of them by acting on the body’s natural pain-regulating system, the endocannabinoid system. This discovery could lead to the development of safer and more effective painkillers in the future.

Paracetamol, known by brand names such as Tylenol, is one of the most widely used pain and fever medications in the world. Despite being widely used for decades, scientists are still trying to understand exactly how it works inside the body.

It was long thought that it worked by reducing the activity of an enzyme called COX2 in the central nervous system, which would help reduce pain and inflammation. However, to achieve this effect, paracetamol would need to be present in very high concentrations, which raises questions about whether this is actually the main way it works.

Another theory suggests that acetaminophen may interact with the so-called endocannabinoid system, a set of natural substances and receptors in the body that regulate various functions, including pain, mood, and appetite. This system is similar to the one activated by compounds in marijuana, but it is produced by the body itself.

Within this system, a substance derived from paracetamol called AM404 may affect how the body deals with pain. This substance appears to prevent the breakdown of a compound called anandamide (associated with feelings of well-being) and even indirectly activate certain pain and temperature-sensitive receptors.

However, the levels of AM404 produced after paracetamol use are very low, which suggests that it may not be strong enough to cause all of these effects on its own.

Recent research suggests that the story may be even more complex. A new study, conducted by scientists at Indiana University, USA, challenges ideas that scientists have long assumed.

Rather than increasing natural pain-relieving substances, as previously thought, paracetamol may actually reduce the production of one of these substances, and surprisingly, this also helps to reduce pain.

This is because acetaminophen appears to inhibit the action of an enzyme called DAGLα, which is responsible for producing a compound called 2-AG, one of the body’s main endocannabinoids. 2-AG normally binds to receptors to regulate pain, but the new study suggests that in some brain circuits, reducing the amount of 2-AG may be what prevents the sensation of pain.

To investigate this, the scientists used a laboratory model with isolated neurons (brain cells) that were cultured to simulate a complete endocannabinoid signaling circuit.

They tested how acetaminophen affected this circuit and found that it did indeed decrease the activity of DAGLα, reducing the amount of 2-AG available. This reduction appeared to decrease the transmission of pain signals.

To confirm this, they also tested another compound that blocks DAGLα, called RHC80267, and found that it also reduced pain, but only in normal mice. In mice lacking CB1 receptors, this effect disappeared, confirming that these receptors are an important part of the process.

These results suggest a new explanation for how paracetamol works. Rather than simply activating the “feel-good messengers,” it may be turning off a specific type of pain-related signal in a specific part of the brain. In other words, by preventing the production of certain endocannabinoids, paracetamol cuts off the “permission” that some circuits in the body give for pain to be felt.

This figure compares two possible ways that paracetamol (or acetaminophen) can relieve pain in the body. On the left side, we have the traditional model, which says that paracetamol is converted in the liver into a substance called para-aminophenol. This substance travels to the brain, where it is converted into another compound called AM404. AM404 blocks a natural substance in the body, anandamide, from being eliminated, increasing its presence. This activates receptors in the brain linked to pain relief. The right side of the figure shows a new discovery: paracetamol can also act in a different way, directly in the brain, by blocking an enzyme that normally helps produce another natural substance called 2-AG. By reducing 2-AG, this pathway also reduces the activation of receptors linked to pain. In other words, paracetamol can relieve pain by both increasing and decreasing certain chemical messengers in the brain, which changes our understanding of how it actually works.

This discovery is important because it completely changes the way we understand this common medicine, and could pave the way for the creation of safer and more effective painkillers in the future.

In addition, this new model helps explain why paracetamol is effective for some types of pain but not others, and why it works differently in different people.

It also highlights the importance of the endocannabinoid system in pain control, which could inspire future research to create drugs that target this system more precisely and with fewer side effects.

This shift in scientific understanding of paracetamol shows how even ancient medicines still have secrets to reveal, and that science never stops investigating.

READ MORE:

Acetaminophen inhibits diacylglycerol lipase synthesis of 2-arachidonoyl glycerol: Implications for nociception

Michaela Dvorakova, Taryn Bosquez-Berger, Jenna Billingsley, Natalia Murataeva, Taylor Woodward,  Emma Leishman,  Anaëlle Zimmowitch,  Anne Gibson,  Jim Wager-Miller,  Ruyi Cai,  Shangxuan Cai, Tim Ware, Ku-Lung Hsu, Yulong Li, Heather Bradshaw, Ken Mackie, and Alex Straiker

Cell Reports Medicine. 102139 May 16, 2025

DOI: 10.1016/j.xcrm.2025.102139

Abstract:

Acetaminophen (paracetamol) is a common analgesic, but its mechanism of action remains unknown. Despite causing around 500 deaths annually in the US, safer alternatives have not been developed. Because endocannabinoids may have a role in acetaminophen action, we examine interactions between the two. We report that acetaminophen inhibits the activity of diacylglycerol lipase α (DAGLα), but not DAGLβ, decreasing the production of the endocannabinoid 2-arachidonoyl glycerol. This gives rise to the counterintuitive hypothesis that decreasing endocannabinoid production by DAGLα inhibition may be antinociceptive in certain settings. Supporting this hypothesis, we find that diacylglycerol lipase (DAGL) inhibition by RHC80267 is antinociceptive in wild-type but not CB1 knockout mice in the hot-plate test. We propose (1) that activation of DAGLα may exacerbate some forms of nociception and (2) a mechanism for the antinociceptive actions of acetaminophen, whereby acetaminophen inhibits a DAGLα/CB1-based circuit that plays a permissive role in at least one form of nociception.