General anesthesia -- the enabler of modern surgery and medical intervention -- is one of the great triumphs of the scientific method. Ether, the compound from which almost all modern anesthetics are derived, was discovered largely by luck and its derivatives through trial and error. As a result, however, much about these drugs remains mysterious. Even today, doctors are baffled as to why exactly anesthetics cause unconsciousness in patients.

For the most part, however, doctors are happy that anesthetics indeed work and seem to be safe. Used at the right doses in healthy people, the drugs rarely cause serious complications or side effects; the risk of death in patients undergoing general anesthesia, for example, is 1 in 250,000. But recent inquiries into how these strange chemicals act on the cellular level have uncovered a troubling long-term possibility: that general anesthetics may potentially contribute to cognitive impairment in vulnerable patients such as the very young and very old.

In 2007, Dr. Zhongcong Xie, now an associate professor of anesthesia at Massachusetts General Hospital, and colleagues published the first in a series of studies demonstrating that commonly used general anesthetics can cause cell death and plaque accumulation in brain cells -- both potential hallmarks of dementia and Alzheimer's disease. More recently, at the Mayo Clinic, in Minnesota, anesthesiologist Dr. Robert Wilder published a study that found a link between exposure to anesthesia and surgery in infancy and learning disabilities later in life. Both doctors have since been approached with inquiries from concerned patients -- but armed only with early data, neither can offer much reassuring advice. "What can I say? We don't have any answers," says Xie. "We have some troubling warning signs, but we cannot conclude that these drugs are causing brain damage in people."