Methenamine breaks down into formaldehyde in an acidic environment. How good of an antimicrobial agent is formaldehyde? Formaldehyde will kill anything. It’s also used embalming fluid. There’s no acquired or inherent resistance to formaldehyde. Where do we have an acidic environment? In our stomach. We don’t want formaldehyde to form in our stomach, yet we are swallowing this oral tablet. That’s why this tablet is enteric coated. It’s also available OTC in very low doses.
If a physician ever tells you to give a half-dose of methenamine or crush it and put it in the nasogastric (NG) tract, as a nurse you must refuse to do that because the enteric coating must not be broken.
The other place that is acidic in the body is the urine. Urine is not heavily acidic but a little bit. All we need is a pH of somewhere in the 6 range and we could kill any organism in the bladder. If the person doesn’t have acidic urine, we could make it acidic by giving them cranberry juice and vitamin C (ascorbic acid). This generally makes it more uncomfortable for the organism to survive. So this combination is given for people who are prone to chronic UTI.
The group of individuals most susceptible/prone to UTI are those with catheters. Unfortunately this drug is of no use to them because if there’s a catheter in the bladder, all the urine is in the bag and not in the bladder.
Recap
- Breaks down to formaldehyde in acidic environment
- Enteric coated to avoid breakdown in the stomach
- Cranberry juice and Vitamin C help acidify urine
- Not useful with indwelling foley catheters
