Patients who take morphine pills for lower back pain may experience changes in the volume of the brain's gray matter in as little as a month, a small study suggests.

In the 30-day experiment, researchers gave 11 patients daily morphine pills and 10 people placebos and then took magnetic resonance imaging (MRI) scans to see if treatment impacted the brain.

The morphine group had gray matter volume declines of about 3 percent in regions of the brain thought to regulate emotions, cravings and responses to pain and volume increases in areas believed to be responsible for learning, memory and executive function. No changes occurred with the placebo.

"Because we are seeing that opioids rapidly change the brain, our take-home message is that opioids should be reserved for cases when most other treatment options have failed," said lead study author Dr. Joanne Lin, a researcher at the University of Alabama at Birmingham.

While more research is still needed to understand how opioids may alter the brain and whether these shifts in gray matter are harmful, caution is still warranted because the medicines can be addictive and lead to abuse and overdose deaths, Lin added by email.

"We know that some people become addicted, but we don't really know why - or how to stop it," Lin said. "By examining brain changes, we may be able to identify targets to prevent adverse opioid events."

While animal research has linked long-term opioid use to changes in the brain, the current study offers fresh insight into how even short-term use of the drugs may alter the human brain, Lin and colleagues note in the journal Pain Medicine.

Patients in the current study had been suffering from chronic back pain for around eight years on average, and none of them had a history of drug abuse or opioid use.

The patients experienced similar rates of pain reduction whether they got morphine or a placebo, though all of them were allowed to take over-the-counter pain drugs during the study period.

Among people in the morphine group, gray matter volume decreased in several reward-processing regions, such as the gyrus rectus, which regulates learning and memory, and the insula, an area involved in cravings.

Shrinkage in the left dorsal posterior cingulate, involved in pain processing, was significantly associated with morphine consumption, and higher doses of the opioid pill were tied to larger volume changes in this part of the brain.

Significant volume loss was also seen in the amygdala, an area central to emotional responses such as fear, anger, pleasure and pain, as well as being involved in impulse control, addiction and tolerance.

For morphine users, gray matter volume also increased in regions located outside reward-processing networks, such as in the cingulate cortex, which is involved many functions, including pain responses and learning.

In addition to the study's small size, another limitation is that the age and gender makeup was markedly different between the two groups, the authors note. Researchers also didn't ask people on placebo if they thought they were taking morphine, so they cannot know if the participants' beliefs affected shifts in brain volume.

While doctors should still weigh the potential negative consequences of opioid use before prescribing these drugs, the study findings on their own don't provide enough evidence to suggest physicians should change how they treat pain, said Laura Stone, a researcher at McGill University in Montreal who wasn't involved in the new research.

Often, a combination of physical therapy and medication may be best for back pain, Stone added by email. Exercise, yoga, meditation and a healthy diet may ease discomfort and also have the potential to address problems that can accompany back pain such as depression and anxiety.

"Morphine and other similar opioid drugs provide excellent pain relief in many individuals, especially for short treatment periods, and can result in real improvements in quality of life," Stone said. "Concerns about the negative consequences of long-term use must be balanced for each individual against the potential therapeutic benefits."

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