If you ever have taken drugs for general pain, headaches, cold, or allergy, chances are that you have taken a chemical called acetaminophen or paracetamol.

Paracetamol (known more commonly as acetaminophen in the North America) is the most widely used painkiller in the world, marketed under different brand names, such as Tylenol in the North America and Panadol in the UK.

Acetaminophen has been in use for over 50 years, but surprisingly, its mode of action as painkiller is still largely unknown.

To know more about Tylenol, naturally, I went to Tylenol homepage and was very surprised at how little information there was about the drug. All it had were merely two lines: “TYLENOL® (acetaminophen) is thought to work by elevating the body’s overall pain threshold, so that you feel less pain. It starts working fast, but won’t irritate your stomach.”

Yeah, I feel less pain, like I didn’t know that… Even worse, this short description is followed by another short description of how non-steroid anti-inflammatory drugs (NSAIDs) work, which are different class of painkillers from acetaminophen.

So, here is your chance of getting to know your painkiller a bit better than just two sentences!

The origin of acetaminophen as a painkiller was by an accident. In the late 19th century, a pharmacist gave the wrong chemical for treating intestinal parasites. Instead of getting rid of the parasites, this chemical reduced the patient’s fever, which interested the doctors. The chemical turned out to be acetanilide (coal-tar derivative) which was used in dye industry and never before fed to humans. Over the years, research led to the development of acetaminophen as an effective and non-toxic painkiller. Marketing of acetaminophen started in 1955 in the US, and it still remains one of the most popular painkillers today.

Acetaminophen is not one of NSAIDs, such as aspirin. Unlike NSAIDs, acetaminophen has no effective anti-inflammatory activity. Also, it does not cause gastrointestinal bleeding as a side effect and can be taken without food without irritating the stomach (which is why Tylenol is often preferred than Aspirin).

After over half a century of usage as painkiller, the mechanism of acetaminophen remains much debated. The general understanding is that the action of acetaminophen takes place predominantly in the central nervous system rather than at the site of inflammation. There may be several mechanisms involved that are interlinked across multiple pathways. In one of the mechanisms, acetaminophen weakly inhibits the first step of prostaglandin biosynthesis. Prostaglandins are pro-inflammatory compounds that cause fever, inflammation, and pain. More specifically, acetaminophen selectively inhibits one of the two forms of cyclooxygenase (COX) enzymes, COX-2. NSAIDs also work by inhibiting COX enzymes, but the selectivity of acetaminophen is a key factor in having little or no toxic effect in gastrointestinal tract. Nonetheless, acetaminophen interacts with other neurotransmitter systems, and the detailed understanding of the drug is still missing.

Do not worry if you didn’t really understand the previous paragraph. It’s just one of the ways scientists say, “We don’t really know how it happens.”

Should we be worried that we don’t really know how acetaminophen works? The drug has been tested and studied vigorously, and it is generally agreed to be safe. However, overdose of acetaminophen is known to cause liver damage. Alcohol consumption will increase this risk, so do not drink alcohol when you are taking acetaminophen! As with any medicine, always read the label carefully and follow the directions from your doctor/pharmacist.


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