Gabapentin contains a single active ingredient: a synthetic compound structurally related to GABA, a natural chemical messenger in the brain that calms nerve activity. Despite this structural similarity, gabapentin does not actually work on GABA receptors. The rest of what’s in a gabapentin pill, capsule, or liquid depends on the formulation, and those inactive ingredients matter if you have allergies or dietary restrictions.
The Active Ingredient
The active compound in every gabapentin product is gabapentin itself. It was designed to mimic the brain’s own calming neurotransmitter (GABA), but it turned out to work through an entirely different mechanism. Gabapentin binds to a specific part of calcium channels on nerve cells, reducing the release of chemical signals that transmit pain and excitatory impulses. Over time, it also decreases the number of these calcium channels present at nerve endings, which further dials down overactive nerve signaling.
This mechanism is why gabapentin is FDA-approved for two conditions: nerve pain after shingles (postherpetic neuralgia) in adults, and as an add-on treatment for partial seizures in adults and children aged 3 and older. Both conditions involve nerves firing excessively, and gabapentin helps quiet that activity.
One notable feature of gabapentin is that your body doesn’t break it down. It passes through unchanged and is eliminated entirely by the kidneys. Its half-life is 5 to 7 hours, meaning the drug clears your system relatively quickly, which is why it’s typically taken multiple times per day.
Inactive Ingredients in Capsules
The brand-name version, Neurontin, lists the following inactive ingredients inside its capsules: lactose, cornstarch, and talc. The capsule shells themselves vary by strength:
- 100 mg capsules: gelatin and titanium dioxide
- 300 mg capsules: gelatin, titanium dioxide, and yellow iron oxide
- 400 mg capsules: gelatin, red iron oxide, titanium dioxide, and yellow iron oxide
The imprinting ink on the capsules contains FD&C Blue No. 2 and titanium dioxide. If you take a generic gabapentin capsule, the inactive ingredients can differ by manufacturer, so check the label or ask your pharmacist.
Inactive Ingredients in Tablets and Liquid
Gabapentin tablets contain a different set of inactive ingredients: cornstarch, magnesium stearate, talc, candelilla wax, purified water, and several binding and coating agents. The tablet formulation does not contain lactose or gelatin, which matters if either is a concern for you.
The oral solution is the simplest formulation. It contains glycerin, xylitol (a sugar alcohol used as a sweetener), purified water, and artificial cool strawberry anise flavor. This version avoids many of the fillers and coatings found in solid forms, making it a potential option for people sensitive to those ingredients.
Allergen and Dietary Concerns
Gabapentin capsules contain lactose, so if you’re lactose intolerant and experience unexplained stomach issues, the capsule form could be contributing. Switching to tablets or the oral solution eliminates that exposure. The capsule shells also contain gelatin, which is animal-derived. This is relevant if you follow a vegan or vegetarian diet, or if you avoid animal products for religious reasons. Tablets and the liquid form are gelatin-free.
None of the standard Neurontin formulations list wheat or gluten-containing ingredients. However, cornstarch (present in both capsules and tablets) is sometimes a concern for people with severe corn allergies. Generic versions may use different fillers entirely, so always verify with the specific manufacturer if you have known sensitivities.
Different Gabapentin Products Are Not Interchangeable
Gabapentin comes in several brand-name versions that are not the same medication in practical terms. Neurontin is the original immediate-release form. Gralise is an extended-release tablet designed specifically for nerve pain after shingles. Horizant takes a different approach entirely: it contains gabapentin enacarbil, a prodrug that your body converts into gabapentin after absorption. This prodrug uses a different transport system in the intestines, which means it gets absorbed more consistently than standard gabapentin, especially at higher doses.
The FDA explicitly states that Horizant is not interchangeable with Neurontin or Gralise. Each version has a different absorption profile, different dosing schedule, and different approved uses. If your prescription is for one specific form, switching to another without guidance could change how much active drug actually reaches your bloodstream.
How Dosing Relates to What’s in Each Pill
Standard gabapentin capsules and tablets come in several strengths, typically 100 mg, 300 mg, 400 mg, 600 mg, and 800 mg. The amount of active ingredient varies, but so do the inactive ingredients. Higher-strength capsules contain additional coloring agents (iron oxides) that give them their distinct appearance, which is how pharmacists and patients can visually distinguish between doses.
For adults with nerve pain after shingles, the typical maximum is 1,800 mg per day. The same ceiling applies for seizure management. Treatment usually starts at 300 mg and increases gradually. Because gabapentin is cleared by the kidneys without being processed by the liver, people with reduced kidney function need lower doses, but liver problems don’t affect how the drug is handled.