To Your Good Health: Clarifying whether natural formaldehyde in the body is toxic
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DEAR DR. ROACH: It’s well-known that formaldehyde is toxic and can be a cause of cancer. Hiprex, which I’ve been taking for a few years, works by turning into an antiseptic called formaldehyde in the urinary tract to prevent bacteria from growing. Should I be concerned? I haven’t read any studies saying that it’s harmful to be taken for the long-term future. — J.B.
ANSWER: You’re exactly right that methenamine (Hiprex and other brand names) in acidic urine will be hydrolyzed to formaldehyde, which is an antiseptic that kills bacteria in the urine. Formaldehyde isn’t an antibiotic, and methenamine isn’t indicated for treating urinary tract infections; rather, it prevents them, as you correctly said.
The urine needs to be acidic for methenamine to become formaldehyde, so you need to be careful to avoid alkalinizing food with your diet. (Citrus fruits, despite being acidic themselves, promote a more-alkaline urine because the liver makes bicarbonate ion out of the citric acid, which turns the urine alkaline.) Or you can take methenamine with an organic acid, such as vitamin C or aspirin, to ensure acidic urine.
The body makes formaldehyde in large amounts daily as part of the catabolism (breakdown) of proteins and amino acids — thousands of times larger than the amount of formaldehyde that’s made in the urine by taking methenamine. Formaldehyde is, indeed, a known carcinogen when it’s inhaled. It isn’t clear that formaldehyde in food or water is carcinogenic.
Rats that were given large amounts of formaldehyde (according to body weight) didn’t have increased cancer rates. More importantly, large studies of people taking methenamine haven’t shown increased cancer rates.
Even with all this reassuring data, it’s still wise to take any symptoms seriously. Blood in the urine could be due to an infection or bladder irritation. But it’s also a possible sign of bladder cancer and should be taken seriously, especially in people who are at a higher risk — such as those who are taking immunosuppressive drugs.
DEAR DR. ROACH: What could cause acute parotitis in someone who’s in their 80s? They’re now on antibiotics after a visit to urgent care. — P.R.
ANSWER: The parotid gland is the major salivary gland. There’s one on each side, and it’s located in the posterior cheek in front of each ear. The most likely diagnosis is acute bacterial parotitis. In an 87-year-old, dry mouth is a common risk factor for an infection, and any dental hygiene problems can also allow bacteria to get into the gland. Bacteria then go from the mouth up into the gland.
Other considerations would be a salivary stone, especially if only one gland is affected. Some medications can cause parotitis, as can some autoimmune diseases — especially Sjogren’s syndrome (which also causes dry eyes). In 2026, we’d also have to be concerned about mumps. Even though a person of 87 was almost certainly infected by mumps at a young age, it’s possible to lose immunity with advanced age. There are periodic outbreaks that are associated with low vaccination rates, so an 87-year-old could potentially be exposed and get sick.
Finally, there are parotid tumors that can mimic an infection. If their symptoms don’t get better with antibiotics, a reevaluation with a primary care doctor or ENT surgeon should be done.
Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.