Not everyone has side effects, but those who do may notice them across these groups (MedlinePlus, Gabapentin):
Central nervous system
Dizziness (especially standing up quickly), drowsiness, confusion or trouble thinking clearly, and headaches — a well-known effect ranging from mild to severe.
Gastrointestinal
Nausea and occasional vomiting, diarrhoea or constipation (severe diarrhoea can rarely lead to dehydration), and abdominal cramps — usually mild, but more noticeable for sensitive stomachs.
Psychiatric and mood
Mood swings, depression (especially in those already at risk), insomnia, and agitation or restlessness.
Neurological
Memory impairment, cognitive dysfunction (thinking, reasoning), speech problems, and abnormal dreams — the vivid "gabapentin dreams" people report are a documented, usually harmless, effect.
Physical
Weight gain, oedema (swelling in the hands, feet, or ankles), fatigue, and — rarely — respiratory depression, a greater concern for people with existing breathing problems.
Skin and other
Skin rash or itching (pruritus), occasional visual disturbances or sexual dysfunction, and — rarely — a serious hypersensitivity reaction that needs urgent care. A new rash is always worth reporting promptly.