Dizziness and somnolence are the most frequent side effects. Fatigue, ataxia, peripheral edema (swelling of extremities), and nystagmus are also common. Gabapentin is associated with a weight gain of 2.2 kg after 1.5 months of use. Case studies indicate that it may cause anorgasmia and erectile dysfunction, as well as myoclonus that disappear after discontinuing gabapentin or replacing it with other medication. DRESS, anaphylaxis, respiratory depression, and increased suicide behavior are the rare but serious side effects.

Suicide

The gabapentin label contains a warning of an increased risk of suicidal thoughts and behaviors. According to an insurance claims database study, gabapentin use is associated with about 40% increased risk of suicide, suicide attempt and violent death as compared with a reference anticonvulsant drug topiramate. The risk is increased for both bipolar disorder and epilepsy patients.[61] Another study has shown an approximately doubled rate of suicide attempts and self-harm in patients with bipolar disorder who are taking gabapentin versus those taking lithium.

Respiratory depression

Serious breathing suppression, potentially fatal, may occur when gabapentin is taken together with opioids, benzodiazepines, or other depressants, or by people with underlying lung problems such as COPD. Gabapentin and opioids are commonly prescribed or abused together, and research indicates that the breathing suppression they cause is additive. For example, gabapentin use before joint replacement or laparoscopic surgery increased the risk of respiratory depression by 30-60%.

Withdrawal and dependence

Withdrawal symptoms typically occur 1–2 days after abruptly stopping gabapentin. Agitation, confusion and disorientation are the most frequently reported, followed by gastrointestinal complaints and sweating, and more rare tremor, tachycardia, hypertension, and insomnia. In some cases, users experience withdrawal seizures.

All these symptoms subside when gabapentin is re-instated.

On its own, gabapentin appears to not have a substantial addictive power. In human and animal experiments, it shows limited to no rewarding effects. The vast majority of people abusing gabapentin are current or former abusers of opioids or sedatives. In these persons, gabapentin can boost the opioid “high” as well as decrease commonly experienced opioid-withdrawal symptoms such as anxiety.

The most common side effects of gabapentin in adult patients include dizziness, fatigue, drowsiness, weight gain, and peripheral edema (swelling of extremities).Gabapentin may also produce sexual dysfunctionin some patients, symptoms of which may include loss of libido, inability to reach orgasm, and erectile dysfunction. Gabapentin should be used carefully in patients with renal impairment due to possible accumulation and toxicity.

An increase in formation of adenocarcinomas was observed in rats during preclinical trials; however, the clinical significance of these results remains undetermined.  Gabapentin is also known to induce pancreatic acinar cell carcinomas in rats through an unknown mechanism, perhaps by stimulation of DNA synthesis; these tumors did not affect the lifespan of the rats and did not metastasize

Gabapentin may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:

• drowsiness
• tiredness or weakness
• dizziness
• headache
• uncontrollable shaking of a part of your body
• double or blurred vision
• unsteadiness
• anxiety
• memory problems
• strange or unusual thoughts
• unwanted eye movements
• nausea
• vomiting
• heartburn
• diarrhea
• dry mouth
• constipation
• increased appetite
• weight gain
• swelling of the hands, feet, ankles, or lower legs
• back or joint pain
• fever
• runny nose, sneezing, cough, sore throat, or flu-like symptoms
• ear pain
• red, itchy eyes (sometimes with swelling or discharge)

Some side effects may be serious. If you experience any of the following symptoms, call your doctor immediately:

• rash
• itching
• swelling of the face, throat, tongue, lips, or eyes
• hoarseness
• difficulty swallowing or breathing
• seizures

Gabapentin may cause other side effects. Call your doctor if you have any unusual problems while taking this medication.

If you experience a serious side effect, you or your doctor may send a report to the Food and Drug Administration’s (FDA) MedWatch Adverse Event Reporting program online [at http://www.fda.gov/Safety/MedWatch] or by phone [1-800-332-1088].

Suicide

In 2009 the U.S. Food and Drug Administration issued a warning of an increased risk of depression and suicidal thoughts and behaviors in patients taking gabapentin, along with other anticonvulsant drugs modifying the packaging insert to reflect this. A 2010 meta analysis confirmed the increased risk of suicide associated with gabapentin use.

Overdose

Persons who accidentally or intentionally ingested overdoses may have drowsiness, sedation, blurred vision, slurred speech, somnolence and possibly death, if a very high amount was taken, particularly if combined with alcohol. Serum gabapentin concentrations may be measured to confirm diagnosis.

The DownSide of Gabapentin

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

• Dizziness, drowsiness, tiredness, fever, and nystagmus (a rapid and uncontrollable movement of the eyes). Drowsiness caused by gabapentin may affect your ability to drive or operate machinery. Alcohol and other drugs with sedative properties enhance this effect.
• May cause behavioral problems, hostility or aggression, or thought disturbances when used to treat epilepsy in children aged three to twelve years.
• Other side effects may include blurred vision, amblyopia (lazy eye), dry mouth, peripheral edema (fluid retention in the feet and hands), tremor, sexual dysfunction, and gastrointestinal disturbances.
• Best titrated up slowly to reduce the risk of side effects; however, this may delay the onset of an effect.
• Similar to other anticonvulsant medicines, gabapentin may increase the risk of depression and suicidal thoughts, particularly in young adults under the age of 24.
• Gabapentin has been associated with a discontinuation syndrome when abruptly stopped. Symptoms include anxiety, insomnia, nausea, pain, and sweating. It should be tapered off slowly under a doctor’s advice.
• The dosage of gabapentin needs to be reduced in kidney disease.
• Rarely, hypersensitivity reactions may occur. Symptoms may include fever, rash, swollen lymph nodes, swollen facial features, or throat swelling.
• May interact with some other medications including antacids, hydrocodone, morphine, and other drugs that cause sedation or dizziness. May also cause false-positive results on some urinary protein tests.
• There have been some reports of gabapentin misuse and abuse, particularly in people with a history of drug abuse. Be alert for this possibility.
• Gabapentin requires three times daily administration because of its short duration of effect. Gabapentin enacarbil (brand name Horizant) only requires once-daily dosing.
• Only effective for partial-onset seizures, not other types of seizure disorders.
• Some branded and generic forms of gabapentin are not interchangeable.