Opioids, a type of molecule that includes painkillers like morphine, may stimulate tumor growth, according to two studies appearing in the latest issue of the journal Anesthesiology. The studies suggest that opioid painkillers may hamper recovery for cancer patients but could also lead to a new possible target for tumor-fighting drugs.

Compared to normal lung cells, cancerous lung cells have five to 10 times as many receptors that, when bound by an opioid, activate a pain relief pathway in cells, according to a report from a group of University of Chicago Medicine scientists.

In the other study, researchers from the University of North Carolina found that breast cancer patients with one copy of a genetic mutation that made them less sensitive to opioids were twice as likely to survive 10 years after treatments.

The two studies contribute to mounting evidence that both naturally-occurring opioids and those found in painkillers can stimulate tumor growth and spread.

Laboratory studies demonstrate that opioids influence tumor progression and metastasis, Jonathan Moss, a University of Chicago Medicine professor and author of a commentary on the two studies, said in a release.

However, in his commentary for Anesthesiology, Moss noted that there have been no controlled studies in humans demonstrating a direct link between opioids in stimulating tumor growth or a direct link between opioid blockers and a reduction in tumor progression.

In the first study, when researchers transplanted human lung cancer cells into mice, they found that cancer cells with more opioid receptors grew more than twice as fast as tumor cells without extra receptors, and were 20 times more likely to spread to other parts of the body.

The scientists did not give painkillers to the mice, so naturally occurring opioids, such as endorphins would have caused any effects related to the extra receptors.

Results from the mouse study suggest that the mu opioid receptor, which mainly binds to morphine, could be a target for therapeutics to stop tumor growth and reduce metastasis, the authors wrote.

In the epidemiological study, researchers examined survival rates for more than 2,000 breast cancer patients and found that chances for survival increased in women who were less sensitive to opioids.

Women with two copies of the opioid-insensitivity mutation were four times as likely to survive over 10 years compared to women without the mutation, according to the study.

The scientists admitted that the lack of treatment data limited their results. Their research could not distinguish between the survival effects of opioids naturally produced by the body and the effects of opioid painkillers.

These studies have caused anesthesiologists to re-evaluate how best to do anesthesia and pain control for cancer patients, Moss said.

Opioids, such as those found in the opium poppy, have been used for pain relief since ancient times, but with drawbacks.

At a meeting of the American Academy of Pain Medicine in February, researchers presented preliminary data showing that patients who took high doses of opioids for long periods of time had hormonal abnormalities, according to Medpage Today.

The study, which has not yet been published in a peer-reviewed journal, examined 22 patients who were on continuous opioid therapy for at least 20 years.

More than half of the patients had low testosterone, and some also had other hormonal abnormalities, including higher levels of the stress-response hormone cortisol and lower levels of follicle stimulating hormone, which is involved in human reproductive processes.