Millions taking a common painkiller could be unwittingly raising their risk of suffering from dementia and memory problems.
That's the warning from researchers in Ohio, who tracked people prescribed gabapentin - often sold under the brand name Neurontin - for more than a decade after their first dose.
The study, published in the Annals of Internal Medicine journal, concluded that those who took the drug for back pain were up to 40 percent more likely to develop dementia than those who did not take the medication.
Patients who used the medication, which costs 50 cents per capsule, were also 85 percent more likely to suffer from mild cognitive impairment, or memory problems that can be a precursor to dementia.
Scientists behind the study were alarmed by the findings and warned that anyone who had taken the drug - the seventh most commonly prescribed in the US - should be psychologically evaluated immediately.
But doctors not involved in the research advised caution, saying the results were observational and did not prove that taking the drug could cause dementia.
Instead, they suggested it could be linked to another factor among the participants, like those on the drug experiencing the worst back pain.
It wasn't clear how the drug may trigger memory problems, but previous research suggests that gabapentin can reduce activity in neurons, which may impact how they communicate.
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Dr Darria Gillespie, an emergency medicine physician who was not involved in the research, told the Daily Mail: 'I would interpret these findings with a lot of caution.
'The fact they found a 40 percent rise suggests to me there could be another factor in the data that they are not accounting for.'
Dr Arthur Jenkins, a neurosurgeon at Jenkins NeuroSpine in New York who was also not involved in the research, added: 'Patients with two plus years of chronic pain have changes visible on MRI, so even patients without any formal brain diagnosis can have subtle brain issues.
'So this may not be a direct causation, but rather an association with patients who are in enough pain to be on this drug.'
About 69 million gabapentin prescriptions are written in the US every year, according to the CDC.
It was approved in the 1990s to treat seizures and shingles, but amid the opioid crisis, doctors started to use the drug off-label as a painkiller.
This study is just the latest to warn of its potential link to dementia, after another paper warned in 2023 that those taking the drug had a 45 percent higher risk.
The drug has previously been linked to dementia, but doctors have disputed the findings, saying the development of the condition could be due to another factor
For the newer Annals of Internal Medicine paper, scientists extracted data on patients from TriNetX, a health research network containing anonymous patient records from 68 healthcare organizations.
A total of 1.4 million patients with a diagnosis of back pain between 2004 and 2014, and at least 10 years' worth of data was extracted.
Of these, 26,000 were found to have been prescribed gabapentin, and in the analysis, they were matched to the same number of similar patients who had not received the drug.
Results showed that those who were prescribed gabapentin six or more times had a 29 percent higher risk of dementia compared to those who did not receive the drug.
And patients who were prescribed the drug 12 or more times had a 40 percent higher risk of developing the condition.
Overall, 1,815 patients who took gabapentin were diagnosed with dementia, equivalent to 7 percent of the group, while 975 patients, or 4 percent, were diagnosed with mild cognitive impairment.
For comparison, among those who did not take the drug, 1,416 patients, or 5.4 percent, were diagnosed with dementia, and 561, or 2 percent, were diagnosed with mild cognitive impairment.
Dr Leah Mursaleen, head of clinical research at Alzheimer's Research UK, said while the study had some strengths, there were serious limitations.
'This study only shows an association between gabapentin prescriptions and mild cognitive impairment or dementia, so we do not know if the medication is directly causing the higher risk,' she said.
'Gabapentin dosage wasn't recorded, and there was no information on how long people were on the medication.
'Because this study only used health records of people with chronic pain, we cannot rule out other factors that might be influencing the findings.'
The risk of suffering from dementia after taking gabapentin was highest among those between 34 and 49 years old, who had a 144 percent higher risk of developing the condition in the decade after taking the drug.
These adults also had the highest risk of developing the condition, at a more than 250 percent higher risk, while those between 18 and 34 years old had the lowest risk.
There were no cases of dementia in the latter group and less than 10 cases of mild cognitive impairment.