Infants under a year old who are given the pain reliever acetaminophen may be more likely to develop asthma and allergic conditions later on, researchers have found.

Friday's issue of the medical journal The Lancet includes three asthma-related studies, on the risks of acetaminophen and how runny noses and wheezing early in life may be linked to asthma in adulthood.

Pain pills
Acetaminophen

Acetaminophen has been known since 1900 but has only been used widely as an analgesic since the 1950s. It is used to relieve occasional bouts of mild pain and to reduce fever. It is recommended for adults and children.

Pros

Unlike Aspirin, acetaminophen does not cause stomach upset or bleeding problems. It is as effective as Aspirin in treating the pain of mild arthritis — as long as the pain is not caused by inflammation. It is as effective as ibuprofen in relieving the pain of osteoarthritis of the knees.

Cons

Side-effects are rare. However, an overdose can cause serious damage to the liver and kidneys. Large doses can be toxic if you regularly consume even moderate amounts of alcohol — three drinks a day. People should not take multiple medicines that contain acetaminophen.

A February 2007 study found that men who took acetaminophen six or seven days a week were 34 per cent more likely to be diagnosed with high blood pressure than men who did not take analgesics.

A September 2008 study found infants under a year old who were given acetaminophen may be more likely to develop asthma and allergic conditions by age six or seven.

Acetaminophen sends an estimated 56,000 people to the emergency room each year, the U.S. Food and Drug Administration said in 2006. About 100 people die each year after unintentionally overdosing on the drug.

Use of acetaminophen should be reserved for children with a fever of 38.5 C or above, according to the World Health Organization.

Ibuprofen

Around since 1972, it's a non-steroidal anti-inflammatory drug (NSAID) used to relieve the pain, stiffness and inflammation that may accompany a number of disorders. It works similarly to ASA – an analgesic as well as an anti-inflammatory – and is effective in reducing symptoms of rheumatoid arthritis and osteoarthritis. Sold generically and under the brand names Advil and Motrin.

Pros

Relieves mild to moderate discomfort of headache, menstrual pain, pain from soft tissue injuries and pain following operations.

A 2007 study by researchers in Ottawa concluded that ibuprofen works best for relieving the pain of broken bones, bruises and sprains in children.

Ibuprofen has fewer side-effects than many other NSAIDS.

Cons

May raise blood levels of lithium, leading to a risk of serious adverse events — including heart attack and stroke. A 2005 British study said the risk of heart attack rose 24 per cent among people taking ibuprofen. A February 2007 study found that men who took NSAIDS — including ibuprofen — faced a 28 per cent higher risk of developing high blood pressure.

Acetylsalicylic acid (ASA)

At least 50 over-the-counter medications contain ASA as the principal active ingredient. The best known is Aspirin. It was developed as a painkiller to treat headaches or the aches and pains of cold and flu.

Pros

In low doses, it can slow down the formation of blood clots. This can be useful for people at risk of stroke or heart attack. Recent studies also suggest Aspirin may help prevent certain cancers, including colorectal, breast and cancers of the throat, mouth and esophagus. Some studies suggest Aspirin may help reduce the risk of developing Alzheimer's disease.

Cons

Aspirin causes stomach problems — including ulcers and bleeding — in some people. Aspirin's anti-platelet activity has also been blamed for hemorrhagic strokes, caused by bleeding into the brain, in some people who use the drug regularly. Aspirin can cause Reye's syndrome, a rare but sometimes fatal complication involving the brain and inner organs, in some children. Another study found that men who took Aspirin were 26 per cent more likely to have high blood pressure.

The hypothesis that acetaminophen, sold under the brand name Tylenol in North America and Paracetamol elsewhere in the world, may be a risk factor for asthma has been around for a decade. It is based on the observation that use of the drug rose at the same time that the prevalence of asthma increased worldwide.

Despite the results, acetaminophen remains the preferred drug for relieving pain and fever for children, the researchers stressed.

"The findings do lend support to the current guidelines of the World Health Organization, which recommend that paracetamol should not be used routinely, but should be reserved for children with a high fever (38.5 Celsius or above)," Dr. Richard Beasley of the Medical Research Institute of New Zealand in Wellington and his colleagues wrote.

Alternative pain relievers such as Aspirin are linked to the risk of Reye's syndrome, a rare but sometimes fatal complication in children. Ibuprofen, sold generically and under the brand names Advil and Motrin, may provoke asthma attacks.

Beasley's team analyzed data from the International Study of Asthma and Allergies in Childhood, looking for risk factors for asthma, hay fever and eczema in more than 200,000 children from 31 countries.

Use of acetaminophen in the first year of life was associated with a 46 per cent higher risk of asthma by the time children reached six or seven years of age compared with those who never took the drug.

Children who took high doses of acetaminophen within the past year had 3.23 times the risk for asthma symptoms and medium doses showed 1.61 times the risk compared with no drug use, the researchers found.

High dose was defined as once a month or more in the past year, while taking acetaminophen at least once per year but less than monthly was considered medium use.

Antioxidant, virus explanations

There are several possible reasons that could explain the association.

"It's well known that paracetamol reduces your antioxidant defences," Beasley said. "In the airways that can lead to inflammation which is the basis of asthma …and [this] may switch your immune system to become more allergic."

Some researchers believe it's actually the viral infections that cause fever that triggers the onset of asthma. And when children have a viral infection, they are more likely to cough, wheeze or get a runny nose that leads parents to reach for medication, said Prof. Andrew Kemp, a pediatric allergy specialist at Children's Hospital Westmead in Sydney.

If the link is proven then the drug could account for 20 to 40 per cent of asthma cases, a significant level in public health terms, Beasley said.

It will take randomized controlled trials to test whether acetominophen in fact causes asthma, the researchers acknowledged.

Besides drugs, parents can help bring down a child's fever by giving plenty of fluids, taking off his or her clothes and giving the youngster a sponge bath. Even a 0.5 C drop in temperature is enough to reduce the risk of complications from a fever, and make the child feel better, said Dr. Ronald McCoy, a spokesperson for the Royal College of General Practitioners in Australia.

The research was funded by the makers of paracetamol and asthma medications.

Childhood roots of asthma

A second study in the same issue concluded that rhinitis, or hay fever, and other allergic reactions that cause a runny nose were also linked to asthma in adulthood.

European researchers monitored 6,461 people aged 20 to 44 in 14 countries who did not have asthma at the start of the eight-year study.

About 1.1 per cent of subjects without rhinitis developed asthma. Among subjects with non-allergic rhinitis, 3.1 per cent went on to have asthma, compared with 4 per cent of people with allergic rhinitis.

"This large prospective study provides strong evidence for an increased risk of asthma in adults with allergic rhinitis, and to a lesser extent non-allergic rhinitis," the study's authors concluded.

As in the acetominophen study, it will take an randomized control trial to show if treating allergic rhinitis helps reduce the incidence of asthma.

The third study concluded the origins of asthma in adults has its roots in early childhood.

Starting with data from 849 infants, researchers from the Arizona Respiratory Center found 181 went on to have asthma after 22 years of follow up.

Children who were wheezing at age six or seven were four times more likely to develop asthma as adults.

"In over 70 per cent of people with current asthma and 63 per cent of those with newly diagnosed asthma at age 22 years, episodes of wheezing had happened in the first three years of life or were reported by parents at age six years," the team wrote.

With files from the Australian Broadcasting Corp.