A widely used over-the-counter medication, taken by millions globally for decades, is now under scrutiny due to potential cancer risks.

Phenazopyridine, commonly sold under brand names like Pyridium and Safrel, is used to relieve symptoms associated with urinary tract infections (UTIs), such as burning, irritation, and discomfort. It works by numbing the urinary tract lining.

However, concerns about its safety have been raised for years, particularly following animal studies. A 2021 study by the U.S. National Institutes of Health (NIH) found that phenazopyridine caused tumors in two types of rodents. Female mice developed both benign and malignant liver tumors, while both male and female rodents showed tumors in the intestines.

While these findings are based on animal studies and don’t prove the drug causes cancer in humans, experts warn that phenazopyridine is “reasonably anticipated to be a human carcinogen.”

In the UK, phenazopyridine is available without a prescription in lower doses, but higher strengths require a doctor’s approval. Potential side effects include dizziness, headaches, and loss of appetite. More serious risks include seizures, breathing difficulties, and unusual bleeding or bruising, according to the Mayo Clinic.

Rita Jew, president of the Institute for Safe Medication Practices, advised against using the drug, stating: “There is no need for this drug.”

UTIs are common, especially among women, with up to 1.7 million in the UK affected by chronic infections. UTIs typically occur when bacteria from the bowel enter the urinary tract through the urethra. Women are more vulnerable due to a shorter urethra, which makes it easier for bacteria to reach the bladder.

Various factors can increase the risk of UTIs, such as sexual activity, certain contraceptives, hygiene habits, and underlying health conditions. Antibiotics are the standard treatment, but alternatives like methenamine and sodium salicylate are also used.

Dr. Steven Goldstein, a gynecologist in New York, said he recommends phenazopyridine to patients while awaiting lab results, though he was unaware of any links to cancer.