Painkiller Tylenol (paracetamol) — What is it, really? Painkiller Tylenol (paracetamol) — What is it, really? – SOMETHING ABOUT SCIENCE

Painkiller Tylenol (paracetamol) — What is it, really?

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.

Open bottles of Extra Strength Tylenol and Ext...

Photo credit: Ragesoss on Wikimedia Commons

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.

3D chemical structure of acetaminophen. Image credit: Rob Hooft on Wikimedia Commons

3D chemical structure of acetaminophen. Image credit: Rob Hooft on Wikimedia Commons

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.

Thank you for reading and see you in a week! 🙂


Potential Analgesic Mechanisms of Acetaminophen
Mechanism of Action of Paracetamol
New insights into the mechanism of action of acetaminophen: Its clinical pharmacologic characteristics reflect its inhibition of the two prostaglandin H2 synthases
What do we (not) know about how paracetamol (acetaminophen) works?

Lynn Kimlicka

I am a scientist-turned writer and editor, who loves to read and write (more than doing experiments). I have a PhD in biochemistry and molecular biology, with a specialization in structural biology. My interests range widely, from life sciences to pop culture and arts to music. I am bilingual in English and Japanese.

23 Responses

  1. Lea says:

    The article provides proven useful to myself.

    It’s really useful and you’re obviously really educated in this area. You get popped my personal eyes in order to varying views on this subject with intriguing, notable and sound content material.

  2. Ali says:

    This article has been very helpful in at least giving me a broad but modern understanding and history of acetaminophen. Thanks for sharing

  3. Morne says:

    Morne van Biljon u15030963
    Very good explanation of how the drug is originated, but a lot of studies were conducted over the last few years and concluded that Tylenol could be a cause of causing asthma on young children. Tylenol might not be the best painkiller and even though it is one of the most used it could be very dangerous because they still don’t know how this drug reacts completely in someone’s body. It might even cause other illnesses that we don’t even now about. U15030963

    • Lynn Kimlicka says:

      Thank you for your comment, Morne, and bringing new studies to attention.

      According to recent studies, such as this one, it seems that association between the exposure to Tylenol during pregnancy or infancy and the increased risk of asthma in children are still inconclusive, largely due to a confounder (a variable that skews the statistics), respiratory tract infections, that many studies didn’t take into account. But, of course, that is not to say the association does not exist. There is no doubt further studies to come will shed clearer light on the matter.

      Just as you say, we still don’t have the complete understanding of this drug, but more and more studies have been published since this article was written over 3 years ago. (In fact, 2,750 papers on Tylenol have been published since, according to a quick search on PubMed, a popular search site for medical literature.) While it is almost impossible to keep up with the development (being a non-specialist in this particular field), I appreciate your thoughts and comments on the recent development on the topic.

      To readers: I can’t stress enough how important it is to consult one’s physician or pharmacist, who should be kept up-to-date on available medications, adverse effects, etc.

  4. Tilana Swartz (u15012702) says:

    Yes, but according to the Isaac study they used 205 000 children from 31 countries that already had asthma and those who don’t. The study consisted on the amount of Tylenol took and the effects that they underwent afterword’s. The results were that the children with asthma will suffer more severe asthma effects and the children without asthma began to develop asthma symptoms. There is also a hypothesis that if children use strong painkillers at a very young stage the risk is 50% higher for them to develop asthma when they are older. (Acetaminophen/Paracetamol – asthma NYT article 2011) But is this enough to prove that Tylenol is a cause for asthma? (u15012702)

    • Lynn Kimlicka says:

      Thank you for the comment, Tilana, and referring to the NYT article. It was a good read and insightful. (For the readers, here is the link to the article:

      Dr. Mahyar Etminan, a researcher at the University of British Columbia, explains the confounder issue mentioned in my previous comment very well:

      Still, Dr. Etminan believes it is not yet clear that acetaminophen itself is responsible for the increasing prevalence of asthma. “Children who take acetaminophen are usually getting it for fever control, and they get fevers because they have viral infections, which on their own are associated with developing asthma later in life,” Dr. Etminan said. “It’s hard to tease out whether it’s the drug or the viral infection.” (quote from the NYT article)

      Back to the question you posed, “But is this enough to prove that Tylenol is a cause for asthma?” It seems to me (who is NOT a specialist in this specific field), there is no definitive conclusion available yet, and many researchers continue to investigate the link (if there is any link). Here is a good title from one of recent studies: “The association between paracetamol use and asthma: causation or coincidence?” (

  5. Colleen Bailey (u15013872) says:

    Thank you for your blog! It was very insightful. Many medical researchers and medical practitioners tend to support this hypotheses, suggesting a link between asthma and Acetaminophen however, there are those who question this relationship due to other external factors that could lead to the prevalence of asthma. Many hypotheses proposed that the increase of asthma prevalence may be due to environmental exposure to “synthetic” materials and/or allergens. These materials irritate the respiratory system- making the respiratory system vulnerable- and causes sensitivity to these materials that could lead to the development of asthma. Some individuals are more likely to have atopy, which is to have a hypersensitivity to some allergens and could have reacted to acetaminophen due to an allergic reaction. Could this be a possible explanation for the prevalence of asthma? (u15013872)

    • Lynn Kimlicka says:

      Thank you, Colleen. I think the researchers haven’t had results they can all agree on mainly because isolating environmental factors, as you mention, is very difficult. Also, children who are given Tylenol may have prior conditions like viral infection or fever. Are environmental and other factors the true cause? Or are they contributing to the higher risk of asthma, together with Tylenol? Or is Tylenol alone enough to increase the risk? I don’t think we have a sure answer yet, but I have faith in science that one day soon, we’ll figure it out!

  6. Morne says:

    Morne van Biljon u15030963
    There are three important aspects to make “trials” more accurate, these aspects are randomisation, blinds and statistical evidence. A few studies that where conducted to prove that Tylenol caused asthma had a great variation or randomisation. More than 12 countries participated and in those 12 countries people were picked at random but the “blinds” were not done at all. Blinds are looking at the evidence that prove your theory and ignore those who don’t. The more accurate the research the more chances it haves to be proven correctly. ( u15030963 )

  7. Tilana Swartz (u15012702) says:

    The research could be done accurately, but there is a very small gap between a precise amount of Tylenol and an over dosage. Any unnecessary amount can cause asthmatic effects that could result in the need of n medical practitioner’s attention. In the case of every participant in scientific studies the person’s exact weight and length should be in consideration relatively to the amount of Tylenol they should use. (u15012702)

  8. Colleen Bailey (u15013872) says:

    The dosage of acetaminophen (Tylenol) taken by the patient can determine whether or not side-effects will be experienced. The dosage should be taken strictly according to the directions given by the practitioner based on age, gender and/or body volume-index, as Tilana mentioned. Individuals tend to consume more acetaminophen when headaches and respiratory tract infections are experienced thus increasing the likelihood of developing asthma. Presently I don’t see how an accurate conclusion can be drawn to explain the relationship between Tylenol and asthma due to the fact that the methodologies used can’t deliver definite results.

  9. Morne says:

    Morne van Biljon u15030963
    Also another hypothesis is that children have a proportionately larger body surface area (BSA) than adults do. The smaller the patient the greater surface area ratio of the surface of the skin. As a result children have a more likely change to be effected by medication than adults do. In the studies conducted like the Isaac study more than 205 000 children participated in the study. In this there is an increasing amount risk for the children to form random symptoms because there immune system is not yet as strong as it would be when they are older. ( u15030963 )

  10. Tilana Swartz (u15012702) says:

    Other medication that is used with Tylenol during the study period should be known and should be tested if it can result in asthmatic effects because of the usage with Tylenol. In the Isaac study (that I mentioned earlier in my comments) the study methodology was near perfect, but they did not bring in account the other medication the children were taking. They also did not include environmental factors like Colleen had mentioned. When taking allot of different medication at once it can easily lead to an over dosage in a certain substance. (u15012702)

  11. Colleen Bailey (u15013872) says:

    Researchers have found a plausible explanation as to why acetaminophen might provoke or worsen asthma: Acetaminophen can reduce the body’s glutathione- a peptide necessary for the repair of oxidative damage. Mothers who took acetaminophen during pregnancy has also been researched to help uncover the link between acetaminophen and asthma. This might be a credible explanation regarding the prevalence of asthma, however, the studies conducted during the first two years of life- where the development of the lungs are crucial- to examine the exposure to Tylenol has been weakened in spite of the results continuously pointing to a link between Tylenol and asthma, when respiratory tract infections were accounted for. (15013872)

  12. Morne says:

    Morne van Biljon u15030963
    Using drugs or medicine during pregnancy can have harmful side effects to the fetus. Even though Tylenol is commonly used for pregnancy during all stages and there are no evidence to sustain that it haves negative effects on the fetus. But what people forget is that caffeine can have very negative effects to the fetus. Some Tylenol also have small doses of caffeine. Caffeine can cause low birth weight that can cause later problems in life such as asthma. In theory it is possible that the Tylenol itself don’t causes asthma but the other substances in Tylenol can cause the asthma effects. That’s why always consult your GP before taking any medication and the right dosage of the medicine.

  13. Tilana Swartz (u15012702) says:

    Tylenol functions by inhibiting the production of the brain prostaglandins, which are chemical substances that sensitize pain and elevate the body temperature regulation set point. This reduces pain and fever. Tylenol is detected in the breast milk of nursing mother and therefore new-born babies are exposed to this drug directly from birth. This increases the change for them to develop asthma because of their weak immune systems and the fact that even a small amount of this drug can count as an overdose. (u15012702)

  14. Colleen Bailey (u15013872) says:

    Studies have shown that asthma affects 1 in every 11 children, yet years of research under funding caused asthma to remain a relative mystery. Doctors believe that these findings should provide reassurance to parents regarding the usage of Tylenol as an effective and safe medication for pain and fever should it be taken according to the manufacturer’s directions.
    Mothers who took Tylenol during pregnancy have been told not to worry, because the current evidence between the drug use and asthma is weak. There is no evidence suggesting that respiratory infections in pregnancy indicate the association between prenatal exposure and asthma. It is, however, wise to advise pregnant women and parents to minimize the use of acetaminophen and not to deviate from the advised dosage. As long as the correct dosage of Tylenol is taken I don’t believe there to be any significant side effects. As Morne said: It is important to consult your GP first.

  15. Morne says:

    Morne van Biljon u15030963
    After reading this blog and all the comments and reply’s I surely know a lot more about Tylenol and it’s properties. Even though there is still no conclusions if Tylenol is good or bad we came closer to realize if this drug is safe. It is only a matter of time when somebody will find the real truth about Tylenol. Thank you Dr Lynn Kimlicka, Tilana Swartz and Colleen Bailey for the research done and knowledge about Tylenol. I THANK you all. (u15030963)

  16. Tilana Swartz (u15012702) says:

    In my experience and research I see no explanation so far in the link between Tylenol and asthma. I want to thank Dr. Lynn Kimlicka for her contribution and also Colleen Bailey and Morne van Biljon in helping me with my understanding about Tylenol and all the science linked to it. Maybe someone will someday find the truth about Tylenol in our comments were we see none. Thank you all again for you time I appreciate it deeply. (u15012702)

  17. Lynn Kimlicka says:

    Thank you all for your comments and discussion on new developments in understanding Tylenol. Science articles (including this one) often get outdated rapidly as new studies continue to be published, so I’m glad that you brought up recent development/questions in the field. If I see a major advancement in the future in this research area, I will post an update in the comments (or maybe write a new post).

  18. Colleen Bailey (u15013872) says:

    Thank you Dr. Lynn Kimlicka for your blog as well as Tilana Swartz and Morne van Biljon for providing more fascinating information regarding this possible link between Tylenol and asthma. It was interesting to learn about more about this medical controversy and to debate about the possible reasons for this connection. I truly enjoyed this blog and hope to read more about these interesting subjects in the near future. (u15013872)

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  2. August 17, 2012

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