Acetaminophen, also called paracetamol in many countries, is one of the world’s most commonly used pain-relieving medicines.
Millions of people take it every day to reduce pain and fever caused by headaches, colds, flu, muscle aches, or after surgery. When taken by mouth at the recommended dose, acetaminophen is considered very safe and has been used for decades.
In hospitals, however, doctors often give acetaminophen in a different way. Instead of tablets or liquid medicine, they sometimes deliver it directly into a vein through an intravenous (IV) drip.
IV acetaminophen works much faster than tablets because it enters the bloodstream immediately. It also allows doctors to give an exact dose and is especially useful for patients who cannot swallow, are unconscious, or are recovering from major surgery.
Although IV acetaminophen has many advantages, doctors have noticed an important side effect. Some patients experience a sudden drop in blood pressure shortly after receiving the medicine.
For healthy people, this may not cause serious problems. However, for patients who are already critically ill, have severe infections, or are recovering from major surgery, a large fall in blood pressure can reduce blood flow to vital organs and may require urgent treatment.
A new study led by researchers at the University of Copenhagen has now uncovered why this happens. The findings help explain a side effect that doctors have observed for many years but did not fully understand.
The researchers found that around six out of every ten critically ill patients develop lower blood pressure after receiving IV acetaminophen. In about one-third of these patients, the drop is serious enough that doctors must give extra treatment to raise blood pressure back to a safer level.
Even so, IV acetaminophen continues to be widely used because its benefits usually outweigh the risks when patients are carefully monitored.
To understand what causes this reaction, the scientists studied how the body processes acetaminophen. When a person swallows a tablet, the medicine first travels through the liver before entering the rest of the body.
This is known as the “first-pass” effect. However, when acetaminophen is given directly into a vein, it bypasses the liver at first and is processed differently. This creates different chemical byproducts that can affect blood vessels.
The research showed that these byproducts influence tiny structures called potassium channels. Potassium channels help control whether blood vessels tighten or relax. When the channels are activated too strongly, the muscles in blood vessel walls relax. As the blood vessels become wider, blood pressure falls.
To test their idea, the researchers carried out experiments in rats. They gave the animals a medicine that blocks certain potassium channels before administering IV acetaminophen.
The results were encouraging. The treated rats experienced a much smaller drop in blood pressure, suggesting that blocking these channels may one day help protect patients from this side effect.
The researchers stressed that these findings should not worry people who take acetaminophen tablets at home. The problem mainly affects acetaminophen given directly into a vein in hospital settings. Oral acetaminophen remains one of the safest pain medicines when used as directed.
The discovery is particularly valuable because IV acetaminophen is frequently used in intensive care units and emergency departments to treat pain and fever.
During periods when hospitals care for large numbers of seriously ill patients, such as during infectious disease outbreaks, understanding how to reduce this side effect could improve patient safety.
The study may also lead to better hospital treatment in the future. If researchers can develop safe ways to prevent this sudden drop in blood pressure, doctors may be able to use IV acetaminophen with even greater confidence in high-risk patients.
The research was led by Thomas Qvistgaard Jepps and published in the journal Atherosclerosis, Thrombosis, and Vascular Biology.
If you care about blood pressure, please read studies about unhealthy habits that could increase high blood pressure risk, and eating eggs in a healthy diet may reduce risks of diabetes, high blood pressure.
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