One of the most popular drugs in the world can potentially do a number on your eyesight, new research suggests.

Scientists in Taiwan studied the medical records of men taking tadalafil, the active ingredient in the erectile dysfunction medication Cialis, and found that when compared to similar people not taking the drug, tadalafil users were noticeably more likely to develop glaucoma over a 5-year span. The results suggest that doctors should be on the lookout for potential vision complications when prescribing tadalafil and similar phosphodiesterase type 5 inhibitors (PDE5is).

“Ophthalmic evaluation and follow-up may be considered, particularly in patients with risk factors for glaucoma,” the authors wrote in their paper, published this month in the British Journal of Ophthalmology.

Not just for ED

PDE5i drugs can open up blood vessels in certain parts of the body, and were originally being developed to treat hypertension and chest pain, but scientists eventually discovered they were especially effective at improving blood flow to the penis and alleviating erectile dysfunction. While these drugs are now synonymous with ED, PDE5is are commonly used for other indications, such as treating lower urinary tract symptoms caused by an enlarged prostate.

Relatively safe as these drugs are, some research has indicated that they might raise the risk of some vision problems, glaucoma included. There are various forms of glaucoma, but the condition is characterized by progressive damage to the optic nerves that can eventually lead to vision loss. This damage is usually linked to fluid build-up and rising pressure in the eyes, though glaucoma can develop even with normal pressure.

According to the authors, studies on the potential link between PDE5is and glaucoma specifically have been limited so far, particularly in people taking these drugs for urinary tract symptoms. Importantly, people with urinary tract symptoms usually take a daily low dose of PDE5is, compared to the higher but more occasional as-needed doses people take for erectile dysfunction.

The researchers analyzed medical records from a large (and anonymized) database of real-world patients. They focused on nearly 37,000 men over 40 taking tadalafil for urinary tract symptoms and compared them to a similar group of men not taking the drug.

Tadalafil use was associated with a 22% higher risk of developing glaucoma over a 5-year period, the researchers found. The drug was also linked to a greater risk of ocular hypertension, which is non-damaging high eye pressure that can be a risk factor for glaucoma.

What does this mean?

Other research has pointed to a link between PDE5i use and vision problems. These added risks appear to be relatively rare overall, and the same seems to be true with glaucoma. In this study, for instance, 3.93% of men over 50 taking tadalafil developed glaucoma over a 5-year-span, compared to 3.16% of non-users (a 0.77% absolute difference). And PDE5is remain a valuable treatment for various conditions, which might even someday include Alzheimer’s.

Still, glaucoma is an irreversible condition, though current treatments can help prevent or mitigate damage to the eyes. So rare as this potential side effect might be, it’s important for doctors to be aware of the risk, especially when prescribing such drugs to people who might already be more susceptible to it. The researchers also argue that further research is needed to better understand exactly how and why these PDE5is can trigger or worsen glaucoma.