Most commercials for prescription drugs these days carry with them an almost comical laundry list of side effects, which pharmaceutical companies are required by law to mention in their television advertisements. But ads for Zovirax (Acyclovir), a common treatment for cold sores and herpes, may soon be required to contain the most absurd disclaimer of them all.

A small percentage of Zovirax users have been found to experience symptoms of a psychiatric condition called Cotard's syndrome, in which sufferers believe they are decomposing, have missing body parts, or are the walking dead, reports New Scientist. That's right, some Cotard's sufferers actually believe they are zombies.

Cotard's occurs in less than 1 percent of patients on Zovirax — so it's a very rare side effect — but the drug is common enough that a significant number of patients have been known to develop this unusual condition. Researchers Anders Helldén, at Karolinska University Hospital in Stockholm, and Thomas Lindén, at the Sahlgrenska Academy in Gothenburg, recently identified eight people with acyclovir-induced Cotard's from Swedish drug databases alone. The scientists were then able to identify the mechanism that causes this debilitating mental illness.

They found that when acyclovir is broken down in the body, it creates a byproduct called CMMG. In most patients, CMMG is removed from the body by the kidneys. But seven of the eight patients from the Swedish study were also experiencing renal failure, which means that CMMG was allowed to accumulate in their bodies.

"Several of the patients developed very high blood pressure," said Helldén, "so we have a feeling that CMMG is causing some kind of constriction of the arteries in the brain."

The good news is that Cotard's was reversible in each of the patients by putting them on dialysis. One woman's recovery was particularly fascinating. Her condition was first identified after she ran into a hospital emergency room screaming, claiming that she was dead. After a few hours of dialysis, she reportedly updated her condition by saying, "I'm not quite sure whether I'm dead any more but I'm still feeling very strange." Four hours after that, she gave doctors a second update: "I'm pretty sure I'm not dead any more but my left arm is definitely not mine." After 24 hours of treatment, she reported no symptoms.

Helldén said the study's results are particularly fascinating because it could give researchers a way to turn Cotard's on and off. Of course, researchers would presumably use such powers to better understand mental health, not for enacting a diabolical plan to make people believe they are zombies. (We can only hope this is not the sinister beginnings of a zombie apocalypse.)

Helldén was reassuring on that front, promising that it would be unethical to create Cotard's in humans. He said that animal experiments would be the target of such research.

In other words, when the apocalypse comes in the form of lab rat zombies, now you'll know the cause.

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