How long does gabapentin take to work? and what side effects can happen on this medication?

Depends on condition: It depends on the condition being treated. For chronic pain it will be about 5 days for noticeable improvement. If seizures are being treated it could take a little longer and dosing might need to be adjusted. There are a lot of potential side effects, but most people tolerate the medication well. You'll get the best information from your pharmacist so talk to him. Feeling groggy is most common.