A drug used to treat erectile dysfunction and enlarged prostate symptoms may raise the risk of glaucoma, a serious eye condition that can cause gradual vision loss, according to new research. 

An international team of researchers analyzed health records from nearly 74,000 men ages 40 and older with lower urinary tract symptoms to examine whether long-term tadalafil use was associated with a greater risk of glaucoma. 

The study, published in the British Journal of Ophthalmology, compared about 37,000 men prescribed tadalafil with a similar group who did not use phosphodiesterase type 5 inhibitors, the drug class that includes Cialis.

Researchers followed the men for up to five years and looked for new cases of glaucoma and high pressure inside the eye, along with whether patients needed treatment for glaucoma.

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Men who used tadalafil were 22% more likely to develop glaucoma than those who did not use the drugs, according to the findings.

They were also 32% more likely to develop high pressure inside the eye and 33% more likely to develop primary open-angle glaucoma, the most common type of the disease.

Dr. Valerie Kattouf, an optometrist at the Illinois Eye Institute who was not involved in the study, told Fox News Digital that glaucoma can be difficult to notice in its early stages.

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“Glaucoma is an eye condition that especially in its early stages often does not present with visual symptoms,” Kattouf said.

As glaucoma progresses, people may gradually lose their side, or peripheral, vision, she said.

The study found that men who used tadalafil were 22% more likely to develop glaucoma than men who did not take the drug. (iStock)

“Clinical findings of glaucoma are best detected in a comprehensive vision evaluation,” Kattouf added.

Kattouf said people with a family history of glaucoma, high pressure inside the eye or severe nearsightedness may face a higher risk. Those who take Cialis daily over a long period of time may also warrant closer monitoring, she said.

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People who regularly use the medication should have a comprehensive eye exam to establish a baseline for their eye health, according to Kattouf. Doctors can check pressure inside the eyes and perform other tests to look for changes that could signal glaucoma.

The researchers noted several limitations to the study.

People who take the medication regularly should get a thorough eye exam to establish a baseline for their eye health. (iStock)

The study was observational, meaning it found a link between tadalafil use and glaucoma but could not prove that the medication caused the condition.

Researchers also could not account for changes in medication use or other health conditions that developed over time.

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The researchers stressed that the findings do not mean people should stop taking tadalafil. Instead, they said, patients who use the medication long term, particularly those with existing glaucoma risk factors, may benefit from regular eye monitoring.

Fox News Digital reached out to Eli Lilly, the manufacturer of Cialis, and the study’s researchers for comment.