Gabapentin 400mg is commonly prescribed for postherpetic neuralgia (PHN), a type of nerve pain following shingles. It works by calming overactive nerves, reducing pain signals. Many patients find relief with this dosage, though effectiveness varies. Common side effects include dizziness, drowsiness, and swelling. Always take it as directed by your doctor. While not a cure, Gabapentin can significantly improve quality of life for PHN sufferers. Consult a healthcare provider for proper dosage adjustments.