People who use prescription antihistamines to relieve allergy symptoms may be more likely than non-users to carry excess pounds, a new study suggests, although the significance of the connection is not yet clear.

In a study of 867 U.S. adults, researchers at Yale University in New Haven, Connecticut, found that prescription antihistamine users were more likely to be overweight or obese than non-users were.

Among the 268 antihistamine users, 45 percent were overweight, versus 30 percent of the 599 study participants not on the medications.

The researchers stress, however, that the findings do not prove that antihistamines are the cause of the extra pounds. This type of study, known as an observational study, can only point to an association between two variables -- in this case, antihistamine use and body weight -- and cannot prove cause-and-effect.

It's possible that some other factor explains the link, according to lead researcher Dr. Joseph Ratliff, a postdoctoral associate in Yale's department of psychiatry.

There have been studies that show allergies and asthma themselves are associated with obesity, he told Reuters Health in an email, so these conditions themselves may have an effect.

Still, in a report in the journal Obesity, Ratliff and his colleagues say the findings point to an important question for future studies to explore.

According to Ratliff, the researchers were interested in looking at the relationship between antihistamines and weight because of what's known about medications called atypical antipsychotics.

Those drugs -- such as olanzapine (Zyprexa) and risperidone (Risperdal) -- are used to treat schizophrenia and other mental illnesses and carry the side effect of weight gain. They also have antihistamine effects.

Histamine is a chemical produced in the body that is best known for its role in promoting the inflammation associated with allergic responses; blocking histamine is a good thing when it comes to relieving hay fever symptoms, for instance.

But cells throughout the brain have receptors for histamine, and the chemical appears to have a hand in a number of physiological functions -- with appetite control and calorie burning being among them.

So in theory, Ratliff explained, antihistamines could contribute to overeating and slower fat breakdown.

The current findings are based on 867 adults who took part in a government health survey in 2005 and 2006. All participants had their weight and height measured, as well as their blood sugar, cholesterol and levels of the blood-sugar-regulating hormone insulin.

On average, antihistamine users had a higher body mass index (BMI) -- at about 31, which falls into the category of obesity. That compared with a BMI of about 28 among non-users, which correlates to being moderately overweight. BMI is a standard measure of weight in relation to height used to gauge obesity.

When the researchers accounted for participants' age and sex, antihistamine use was linked to a 55 percent increase in the odds of being overweight versus non-use. The medications were not linked to higher odds of elevated blood sugar, insulin or cholesterol, however.

More studies are needed to see whether antihistamines do in fact have an effect on body weight.

The question is important, Ratliff and his colleagues note, as an estimated 50 million Americans have allergies, and anywhere from 35 percent to 50 percent of them use antihistamines.

For now, Ratliff recommended that people with allergies talk with their doctors about all the potential side effects of the different treatment options, and try to find one that works best for them.

Other allergy treatments include corticosteroid nasal sprays and eye drops, decongestants, cromolyn sodium nasal sprays and allergy shots. Non-drug tactics to managing allergies include limiting exposure to symptom triggers -- such as pollen, mold or pet dander -- and using salt-water nasal washes.

SOURCE: link.reuters.com/dyr27n Obesity, online August 12, 2010.

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