Dear Tom and Ray:

You mentioned in a recent column that ethylene glycol (antifreeze) is toxic to animals. Is it also toxic to humans? I'm a doctor, and I prescribe a medicine for my patients that cleans out their colon called Go-Lytely (which makes a person do anything but go lightly). The main ingredient in Go-Lytely is polyethylene glycol, which sounds an awful lot like ethylene glycol, or antifreeze. So, when I prescribe Go-Lytely to prepare my patients for a colonoscopy, am I really prescribing antifreeze? If so, could I just tell my patients to down a gallon of Prestone and save themselves a trip to the pharmacy?

-- Dan

TOM: Well, sending off your colonoscopy patients with a bottle of Prestone is contra-indicated, Doc. I wouldn't recommend it.

RAY: Although, wait a minute. Antifreeze DOES have rust inhibitors. Maybe they'll keep the arteries clean?

TOM: You won't need arteries if you ingest antifreeze. The two substances ARE related (they're polymers, as we chemists say), but they have very different effects on the human body. Polyethelyne glycol -- the stuff you give your patients -- is a very large molecule, too big to be easily absorbed by the body. Its job is to block the absorption of water.

RAY: So when your patients drink 17 gallons of water with their Go-Lytely, as instructed, all that unabsorbed water acts as a "flush," which is what your patients end up doing all night before their procedure.

TOM: Ethylene glycol (antifreeze), on the other hand, is a very small molecule that IS absorbed by the intestines. And according to Dr. Arthur J. Berman, assistant professor of clinical medicine at Columbia Medical School and a Go-Lytely prescriber himself, if you ingest antifreeze, getting your annual colonoscopy is the last thing you'll have to worry about.

RAY: He tells us that in humans, ethylene glycol is metabolized into oxalic acid, a poison that affects the brain, the heart and -- if you live long enough -- the kidneys.

TOM: So, Prestone is not a substitute for Go-Lytely. On the other hand, I'm eager to find out whether Go-Lytely can be a substitute for Prestone. So if you'll write me a prescription, I'll throw a gallon in my trunk. And next time I overheat, I'll pour it in the radiator and ask my health insurer for reimbursement.