Acetaminophen, also called paracetamol, is one of the most commonly used pain-relieving medicines in the world.
Millions of people take it every day to ease headaches, muscle pain, toothaches, fever, and many other common health problems. When taken by mouth at the recommended dose, it has a long history of being both safe and effective.
However, hospitals often give acetaminophen in a different way. Instead of tablets, doctors sometimes deliver it directly into a patient’s vein through an intravenous (IV) drip.
This method is especially useful for people who are seriously ill, have recently had surgery, or cannot swallow medicine because they are unconscious or too sick. Giving the medicine through a vein also allows it to work more quickly and lets doctors control the exact amount that reaches the patient.
Although intravenous acetaminophen has many benefits, doctors have known for years that it can sometimes cause a sudden drop in blood pressure. This effect is usually temporary, but it can be dangerous for patients who are already critically ill.
In intensive care units, keeping blood pressure stable is very important because low blood pressure can reduce the flow of oxygen-rich blood to vital organs such as the brain, heart, and kidneys. Previous reports showed that this side effect was common, but no one fully understood why it happened.
Researchers at the University of Copenhagen decided to investigate this long-standing medical mystery. Their work has now provided an important explanation. The team found that around six out of every ten critically ill patients experience a noticeable fall in blood pressure after receiving intravenous acetaminophen.
About one-third of these patients need medical treatment to bring their blood pressure back to a safe level. Even healthy people who receive the medicine through a vein can sometimes experience the same effect, showing that the problem is not limited to people with severe illness.
The scientists discovered that the body handles acetaminophen differently depending on how it is given.
When people swallow a tablet, the medicine first travels to the liver, where much of it is processed before it enters the rest of the body. This normal route changes the way the drug is broken down.
When acetaminophen is given directly into a vein, it bypasses this first step in the liver. As a result, different leftover substances are produced as the medicine is processed in the body.
The researchers found that these leftover substances affect tiny structures called potassium channels. These channels help control whether blood vessels become tighter or more relaxed.
When the channels are influenced by the by-products of intravenous acetaminophen, the walls of blood vessels relax more than usual. As the blood vessels widen, blood pressure falls. This explains why some patients experience a rapid drop in blood pressure soon after receiving the medicine.
To test whether this explanation was correct, the research team carried out experiments in rats. They used medicines that block certain potassium channels before giving intravenous acetaminophen.
The results were encouraging. Blocking these channels greatly reduced or even prevented the sudden fall in blood pressure. Although more research is needed before this approach can be used in hospitals, the findings suggest that doctors may one day be able to prevent this side effect in patients who need intravenous acetaminophen.
The researchers stress that these findings should not worry people who use acetaminophen at home. The study does not suggest that ordinary tablets are unsafe when taken as directed. Instead, the research focuses on intravenous acetaminophen, which is mainly used in hospitals under close medical supervision.
For most people treating everyday pain or fever, acetaminophen remains one of the most widely recommended medicines when used according to the instructions.
The study is also important because intravenous acetaminophen has become a common treatment in hospitals for managing pain and fever in patients with serious illnesses, including during the COVID-19 pandemic. Understanding exactly why blood pressure sometimes drops will help doctors better monitor patients and may lead to safer treatment in the future.
The research was led by Thomas Qvistgaard Jepps from the University of Copenhagen and was published in the journal Atherosclerosis, Thrombosis, and Vascular Biology.
By uncovering the biological reason behind this well-known side effect, the study gives doctors valuable new knowledge and opens the door to new ways of protecting patients while allowing them to continue benefiting from an important and widely used medicine.
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