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.

Does gabapentin cause weight gain?

Gabapentin may cause weight gain, but it is a rare side effect. Studies have shown that a small number of people taking gabapentin, a drug used to treat epilepsy and postherpetic neuralgia, experienced weight gain. People who do gain weight may gain about 5 pounds after 6 weeks of use.

• In people with epilepsy, weight gain occurred in 3% of people older than 12 years of age who were taking gabapentin (compared to 2% of people taking the placebo). Weight gain was also seen at a similar rate in pediatric epilepsy patients who were 3 to 12 years old.
• In people with postherpetic neuralgia, 2% of patients taking gabapentin experienced weight gain. No weight gain was found among people taking the placebo.

The cause of weight gain with gabapentin is likely due to increased appetite. You may be hungry more often. In some cases, weight gain may be due to fluid retention, another side effect of gabapentin. Another possible cause is not getting enough exercise if gabapentin is causing fatigue.

Some ways to avoid weight gain include:

• Eating a healthy and balanced diet
• Eating smaller portion sizes
• Avoiding high-calorie snacks and desserts like chips, pastries and sweets
• Eating low-calorie snacks like fruits and vegetables to manage hunger
• Getting regular exercise

Swelling from fluid retention may be reduced by:

• Sitting with your feet raised
• Avoiding standing for long periods of time

If gabapentin is causing you to gain weight, do not stop taking this drug on your own. Stopping the drug suddenly can lead to serious problems, especially if you are taking gabapentin for seizures. Abruptly stopping a seizure medicine can cause seizures that won’t stop.

Weight gain is one of many possible side effects. The most common side effects with gabapentin include:

• Clumsiness
• Viral infection
• Sleepiness
• Nausea and vomiting
• Speaking difficulties
• Tremor
• Swelling, usually involving the legs and feet
• Fatigue
• Fever
• Movements that are jerky
• Coordination difficulties
• Double vision
• Unusual eye movement