Elmiron treats interstitial cystitis (IC), a condition that affects more women than men and causes pain and pressure in the bladder. Most Elmiron side effects are mild and stop after the body gets used to the medication. Most people tolerate the medication well.

In clinical trials, the longest that researchers followed trial participants was for a year.

Researchers studied Elmiron side effects in 2,627 patients, according to the drug’s prescribing information. Common side effects occurred in 1 to 4 percent of participants. Thirty-three patients, about 1.3 percent, suffered serious side effects. Six patients died, but other health problems appear to have caused the deaths.

The most common side effect reported was hair loss, or alopecia. Hair loss began in the first four weeks of treatment. About 97 percent of patients who reported cases of alopecia experienced hair loss on a single spot. This usually went away after patients stopped using the drug and hair grew back.

As of June 2020, Elmiron’s prescription information does not list weight gain as a side effect.

However, an October 2017 patient flyer says that “weight gain and swelling caused by fluid buildup have been reported in patients taking Elmiron.”

In animal study data, mice who took higher doses of the drug had a “significant increase (40%) in body weight gain,” according to a March 2016 assessment report of Elmiron by the European Medicines Agency.

According to the drug’s prescribing information, rare side effects happened in one percent or less of the trial participants.

In a phase 4 study of 128 patients, 8 patients (6.3 percent) who took the daily 300 mg dose experienced rectal hemorrhage. Most patients described it as mild. Fifteen percent of patients who took a 900 mg dose had a higher incidence of rectal bleeding.

In a phase 2 study, 6 out 51 patients (11.8 percent) had increases in their liver function tests with a daily dose of 900 mg.

In June 2020, Janssen Pharmaceuticals added a warning for retinal pigmentary changes, also known as pigmentary maculopathy, associated with long-term Elmiron use. Increased dose and length of use appear to be risk factors.

The cause of the disease is not clear. Most cases happened in people who used the drug for three years or longer, but people who used the drug for shorter durations have also reported symptoms.

The prescribing information recommends vision tests within six months of starting treatment and periodic tests after that. Some people continued to have progressive vision damage after stopping treatment.

People who experienced vision damage filed Elmiron lawsuits against Janssen Pharmaceuticals. Lawsuits claim the drugmaker failed to warn them about the risk.

A search of the postmarketing data available on the FDA Adverse Event Reporting System (FAERS) Public Dashboard for “Elmiron” and “pentosan polysulfate sodium” from 1997 to June 30, 2020, showed 2,570 adverse event reports.

Of those reports, 1,748 were serious, including 24 deaths. Data on FAERS comes from voluntary reports and the FDA cannot verify if the adverse event was related to drug use. There were 202 eye disorders reported. These included maculopathy, visual impairment and blurred vision.