Gabapentin is frequently used off-label due to the fact that it is considered to have a low potential for abuse and is regarded as non-addictive.  Due to the fact that the drug elicits both analgesic and anticonvulsant effects, it is sometimes preferred by those undergoing various types of surgery.  It reduces preoperative anxiety via its mechanism acting on GABA ergic neurotransmission, and provides postoperative pain relief.

Gabapentin is an effective treatment for neuropathic pain, seizures, and various off-label conditions – many people experience unwanted side effects.  One unwanted side effect that has been reported in a small percentage of users is weight gain.

It is estimated that approximately 3% of all users will experience some form of weight gain.  Most people won’t notice any significant fluctuations in body weight throughout their treatment.  For this reason, Gabapentin is often referred to as a “weight neutral” drug.

there are some studies highlighting the fact that weight gain can occur on Gabapentin, especially when taken at high doses.  In one study of 28 patients taking 3000 mg per day of Gabapentin, 10 patients gained approximately 10% of their bodyweight.  Despite this finding, the majority of patients remained weight neutral, and some even lost weight (3 patients).

The most common side-effects of Gabapentin  listed are these:

Back pain; changes in vision (double or blurred vision); clumsiness; constipation; diarrhea; dizziness; drowsiness; dry mouth; nausea; stomach upset; tiredness; vomiting; weight gain.

Incidentally, these side-effect listings must, by law, include all effects noted by anyone during any of the trials on the way to becoming a drug. That’s why we see strange listings like the one above, where constipation and diarrhea are listed, side by side!

I don’t know the mechanism of the gain, but suspect it’s mostly water retention, since the weight usually fluctuates a lot. An interesting fact about Neurontin is that it is not metabolized by the body at all – nearly all of the unchanged drug is excreted in urine. This is good news for folks taking a lot of it, since if a drug isn’t metabolized, it won’t cause liver or kidney damage.

Drug action and drug metabolism are unrelated, unless metabolism converts a molecule to an active drug in the body (making that molecule a “prodrug”

The body’s job is to get rid of anything that’s not normally there – especially small molecules like drugs. Metabolism is designed to make things easier to excrete, but Neurontin is already easily excreted due to its structure, so the body need do nothing to assure its excretion.

Digestive Side Effects

It is unclear why a small percentage of people lose weight when taking Neurontin, but it may be related to the small percentage who experience unpleasant digestive effects. In the studies highlighted at DailyMed, up to 5.7 percent of adult and adolescent participants experienced diarrhea, up to 3.9 percent nausea, up to 3.3 percent vomiting and up to 2.2 percent heartburn. In children ages 3 to 12, 8.4 percent experienced nausea, some with vomiting.