LUBBOCK, Texas—A slice of cool, fresh watermelon is a juicy way to top off a Fourth of July cookout and one that researchers say has effects similar to Viagra—but don’t necessarily expect it to keep fireworks going long into the night.

Watermelons contain an ingredient called citrulline that can trigger production of a compound that helps relax the body’s blood vessels, similar to what happens when a man takes Viagra, said scientists in Texas, one of the nation’s top producers of the seedless variety.

Found in the flesh and rind of watermelons, citrulline reacts with the body’s enzymes when consumed in large quantities and is changed into arginine, an amino acid that benefits the heart, and the circulatory and immune systems.

“Arginine boosts nitric oxide, which relaxes blood vessels, the same basic effect that Viagra has, to treat erectile dysfunction and maybe even prevent it,” said Bhimu Patil, a researcher and director of Texas A&M’s Fruit and Vegetable Improvement Center.

Todd Wehner, who studies watermelon breeding at North Carolina State University, said anyone taking Viagra shouldn’t expect the same result from watermelon.

The nitric oxide also can help with angina, high blood pressure and other cardiovascular problems, according to the study paid for by the U.S. Department of Agriculture.