A new British study, appearing in this week's issue of The Lancet, showed a statistically significant increase in colorectal cancer among people who took non-synthetic pituitary growth hormone between 1959 and 1985.
However, the absolute number of cancer cases was small, and researchers say there's no proven biological reason for such a link, if it's indeed real. They also note that, since the mid-1980s, doctors have been prescribing a synthetic form of growth hormone that may have different effects on the body.
What's more, doctors now administer the drug at doses designed not to exceed age-appropriate thresholds, and patients are carefully monitored to make sure their blood levels of the substance stay in this range.
Still, experts say the results deserve further investigation, though they're not reason enough to stop growth hormone treatments in patients who need the substance. The potential link to cancers should give pause to older adults considering growth hormone injections to slow the aging process, an increasingly common but scientifically unfounded phenomenon. It should also be a strong warning to athletes who take extreme doses of the drug as a workout aid.
The U.S. Food and Drug Administration has approved the therapy only for children and adults whose pituitary glands don't make enough growth hormone.
However, as much as 50 percent of the growth hormone on the market goes for unapproved uses, says Dr. Michael Pollak, a cancer specialist at McGill University in Montreal and co-author of an editorial accompanying the journal article.
"If the dose you take achieves normal levels, you have nothing to worry about," Pollak says. "But if you're not really growth-hormone deficient and your growth hormone therapy is giving you unnaturally high levels, then this may be an important warning for you."
Since the 1950s, more than 100,000 people worldwide have received growth hormone supplements.
Dr. Anthony Swerdlow, an epidemiologist at the Institute of Cancer Research in Surrey, England, and his colleagues looked at cancer rates and deaths among 1,848 Britons treated with human growth hormone between 1959 and 1985. All but 1 percent were under the age of 19 when they began the therapy.
By the end of 2000, people who'd received the hormone were nearly three times as likely to have died of cancer as those in the other group, with 10 deaths when about three would have been expected.
Their risk of dying from colorectal tumors or Hodgkin's disease, which strikes the body's lymph system, was 11 times greater, with two deaths each from the maladies. The incidence of colorectal cancer was roughly eight times higher than that in the general population, too, with two cases where just 0.25 would have been predicted in such a young group.
Some conditions for which people take growth hormone may increase their risk of cancer. Yet, even after Swerdlow's team accounted for this effect, they continued to see an elevated risk of colorectal cancer and Hodgkin's disease, as well as deaths from the two conditions.
Swerdlow says the abnormally high rate of Hodgkin's disease among people who used growth hormone "could well be chance." No other work has found reason to connect the two.
However, other scientists have found a potential link between high levels of a growth-promoting molecule, called IGF-1, and certain cancers. Since growth hormone works by stimulating IGF-1, which might explain the higher odds of colorectal tumors in the people who received the therapy.
Dr. Shlomo Melmed, a growth hormone expert at Cedars-Sinai Medical Center in Los Angeles, calls the British findings "extremely tenuous," and adds they don't apply to modern growth hormone therapy.
Before 1985, the substance was pulled from the pituitary glands of cadavers and was prone to contamination, Melmed explains. "We just don't know what those impurities were" or what, if any, harm they might have caused, he says. Today's synthetic growth hormone is not only pristine, but it's administered in strictly controlled doses that stay within the normal range, he adds.
Moreover, although growth hormone can cause tumors to grow faster, there's no evidence that it causes them to appear. A large study of people with acromegaly, whose excessive growth hormone production causes them to become giants, found no increase in the risk of cancer, Melmed says.
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