An allergic reaction can range from annoying to potentially life-threatening, and women tend to have more of the latter kind of reactions than men, which puzzled scientists until now.

Allergic reactions can be as mild as a rash or a little bit of swelling, but sometimes they can result in difficulty breathing, a heart attack, or even death. Women tend to be more prone to this, a new research from the National Institutes of Allergy and Infectious Diseases suggests that the female hormone estrogen may be to blame, according to the Washington Post.

They looked at female mice and noticed that they tended to have more severe and longer-lasting reactions than their male counterparts. Examining the issue deeper, researchers found that estrogen tended to boost activity of an enzyme that lines the blood cells, which can lead to severe allergic reactions.

It does this by producing nitric oxide, which causes a drop in blood pressure and lets fluid from blood vessels leak into tissues, which can lead to dangerous swelling. By blocking that activity, scientists found that this tendency disappeared.

While this may provide tremendous insight into the issue, the study was performed on mice, so more work will have to be done before it can be proved that it is the same situation in humans.

But it could help improve treatment in the future and raise awareness of certain substances, such as those that increase production of nitric oxide.

Dean Metcalfe, head of the institute’s Laboratory of Allergic Diseases and the author of the study, said that researchers “need to be better at associating diseases with gender.”

One supplement that women commonly take as an athletic performance supplement is known as L-arginine, which boosts nitric oxide production. Metcalfe said that in the future, a doctor should ask a woman suffering from anaphylaxis if she is taking health food supplements such as L-arginine.