By Pat Anson

Over the years, gabapentin (Neurontin) has been prescribed for dozens of health conditions, from epilepsy and fibromyalgia to depression and post-operative pain. It’s even been used to treat bipolar disorder. Gabapentin has been marketed for so many different conditions – at times illegally -- that a pharmaceutical company executive infamously referred to the drug as “snake oil.”

Even though it’s been approved for medical use for over 30 years, the UK’s National Health Service admits it’s still “not clear exactly how gabapentin works.”

A new study may finally help explain why gabapentin is an effective pain medication for some patients and an addictive drug with unwelcome side effects for many others.

It could be all in the genes.

Researchers at the University of Edinburgh took another look at a previous study of women with chronic pelvic pain to see why gabapentin worked no better than a placebo for most, but was a moderately effective pain reliever for about 40% of them.

Researchers took saliva samples from 71 women who participated in the study -- 29 who responded to gabapentin and 42 who had no improvement -- and found that the responders were more likely to have a variation of the gene Neuregulin 3 (NRG3). The gene is primarily expressed in the brain, spinal cord and central nervous system, and helps regulate pain sensation and transmission.

The findings, recently published in the journal iScience, may explain why gabapentin works for some women with chronic pelvic pain.

"A genetic factor that can predict how well gabapentin will work in patients offers the prospect of tailored treatment, and provides invaluable insights into understanding chronic pain. We hope eventually to use this genetic marker to optimize personalized treatment decisions and minimize adverse effects for women with chronic pelvic pain," wrote lead author Scott Mackenzie, MD, from the University of Edinburgh's Centre for Reproductive Health.

The study also has implications for other chronic pain conditions. Further research is needed to confirm the findings, but researchers say a genetic test for NRG3 could help limit the use of gabapentin to people who actually benefit from the drug.

"Isolating this single genetic marker is an important discovery that could ultimately help refine treatments for millions of women worldwide who suffer from chronic pelvic pain, as well as increasing our understanding of its role in other pain conditions. We believe this is an exciting opportunity for collaboration with a commercial partner who can help translate the research into a clinical setting," said Susan Bodie, PhD, Head of Business Development for the College of Medicine and Veterinary Medicine at the University of Edinburgh.

Gabapentin and other nerve drugs like pregabalin (Lyrica) have come under increased scrutiny in the UK because they are increasingly involved in overdose deaths.  

A recent analysis of drug tests suggests that gabapentin is also being misused in the U.S. Gabapentin was found in in over 13% of urine samples that tested positive for fentanyl -- about twice the number of drug tests in which prescription opioids were found.

Despite the risks of side effects and addiction, gabapentin is increasingly prescribed “off-label” for conditions it is not approved to treat, such as dental pain. A 2019 study found little evidence that gabapentin and pregabalin should be used for pain and said their effectiveness was often exaggerated by prescribing guidelines.

The CDC’s revised opioid guideline says gabapentin and pregabalin can have “small to moderate improvements” on pain, but with a moderate risk of side effects.