Gabapentin side effects: what you need to know

The most frequently reported side effects of gabapentin are neurological in nature and tend to be dose-related.

They typically appear early in treatment and often improve as the body adjusts.

Very common (affecting more than 1 in 10 people):

• Drowsiness and somnolence: this is the most widely reported effect, particularly at higher doses. It can impair driving ability and concentration
• Dizziness: often worst during the first week and during dose increases
• Fatigue: a general sense of tiredness that many patients describe in the early weeks

Common (affecting 1 in 10 to 1 in 100 people):

• Headache: usually mild and self-limiting
• Nausea and vomiting: tends to settle within the first fortnight
• Tremor: fine tremor of the hands is occasionally reported
• Blurred vision or diplopia: visual disturbances are generally transient
• Dry mouth: can be managed with regular sips of water

These effects are listed in the BNF and patient information leaflet. If they persist or worsen, a dose adjustment may be appropriate.

Your prescriber can advise on whether to continue, reduce or gradually withdraw the medicine.

Weight gain is a recognised side effect of gabapentin, affecting a notable proportion of patients on longer-term treatment. The BNF lists increased appetite and weight gain as common adverse effects.

Why does gabapentin cause weight gain?

The mechanism is not fully understood, but several factors are thought to contribute:

• Increased appetite: many patients report feeling hungrier, particularly for carbohydrate-rich foods
• Reduced physical activity: drowsiness and fatigue may lead to a more sedentary lifestyle
• Fluid retention: peripheral oedema (swelling of the ankles and feet) can contribute to apparent weight increase
• Metabolic effects: gabapentin may influence lipid and glucose metabolism in some individuals

Studies suggest that weight gain is more common at higher doses (above 1800 mg daily) and during prolonged courses.

The average weight gain reported in clinical trials was approximately 2 to 3 kg over several months, though some patients gain more.

Practical tips:

• Monitor your weight regularly from the start of treatment
• Maintain a balanced diet and avoid high-calorie snacking
• Stay as active as your condition allows
• Discuss any significant changes with your prescriber, who may consider alternative treatments if weight gain becomes problematic

Serious side effects of gabapentin are uncommon but important to recognise. Prompt medical attention may be needed.

Respiratory depression:

The MHRA has issued safety warnings about the risk of respiratory depression with gabapentinoids, particularly when combined with opioids, benzodiazepines or in patients with compromised respiratory function.

Symptoms include slow or shallow breathing. This risk led to gabapentin being reclassified as a Class C controlled substance in the UK from April 2019.

Suicidal thoughts and behaviour:

Anti-epileptic medicines, including gabapentin, carry a small increased risk of suicidal ideation.

The MHRA advises that patients and carers should be alert to changes in mood, behaviour or thoughts of self-harm, especially in the early weeks of treatment or during dose changes.

Severe skin reactions:

Rarely, gabapentin may cause Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS syndrome) or Stevens-Johnson syndrome.

Seek urgent medical advice if you develop a widespread rash with fever, swollen lymph nodes or feeling generally unwell.

Angioedema:

Swelling of the face, lips, tongue or throat has been reported rarely. This requires immediate medical attention.

Contact your GP or call NHS 111 if you experience any worrying symptoms. Call 999 if you have difficulty breathing or signs of a severe allergic reaction.

Gabapentin can affect mood and cognitive function in some patients. These effects are important to be aware of, especially if you are taking gabapentin for chronic pain or anxiety-related conditions.

Reported neuropsychiatric effects include:

• Emotional lability: mood swings or irritability
• Confusion and cognitive impairment: difficulty concentrating or forming thoughts clearly, sometimes described as "brain fog"
• Anxiety and nervousness: paradoxically, gabapentin can sometimes worsen anxiety in certain individuals
• Depression: low mood should always be reported to your prescriber
• Memory difficulties: short-term memory may be affected, particularly at higher doses
• Hostility or agitation: uncommon but recognised, especially in children and adolescents

These effects are more likely at higher doses and in older adults. The NICE guidelines on neuropathic pain recommend regular review of patients on gabapentin to monitor for neuropsychiatric symptoms.

If you notice changes in your mood, thinking or behaviour, speak to your doctor. In most cases, symptoms resolve when the dose is reduced or the medicine is gradually withdrawn.

Many gabapentin side effects can be minimised with practical strategies. Working with your prescriber to find the right dose is key.

Reducing drowsiness and dizziness:

• Start at a low dose and increase gradually, as recommended in the BNF
• Take the larger portion of the daily dose at bedtime
• Avoid driving or operating machinery until you know how gabapentin affects you
• Avoid alcohol, which worsens sedation

Managing weight gain:

• Keep a food diary to identify changes in eating habits
• Choose nutrient-dense foods over empty calories
• Aim for regular physical activity appropriate to your condition

Addressing gastrointestinal symptoms:

• Take gabapentin with food to reduce nausea
• Stay well hydrated
• Eat smaller, more frequent meals

General guidance:

• Never stop gabapentin abruptly, as this can cause withdrawal symptoms including anxiety, insomnia and seizures
• Attend regular reviews with your prescriber
• Report any new or worsening symptoms promptly
• Keep a symptom diary to discuss at appointments

NICE recommends that treatment should be reviewed regularly and continued only if there is clear benefit.

Certain groups of patients are more likely to experience side effects from gabapentin and may need closer monitoring or dose adjustments.

Older adults:

Elderly patients are more susceptible to drowsiness, dizziness and falls. The BNF advises starting at lower doses in this group.

Renal function should be checked, as gabapentin is cleared by the kidneys.

Patients with kidney disease:

Gabapentin is excreted unchanged by the kidneys. Reduced renal function leads to higher blood levels and a greater risk of side effects.

Dose reduction is essential, and the BNF provides specific dosing guidance based on creatinine clearance.

Patients taking opioids:

The combination of gabapentin with opioid analgesics significantly increases the risk of respiratory depression, sedation and death. The MHRA strongly advises caution with this combination.

Patients with respiratory conditions:

Those with COPD, sleep apnoea or other conditions affecting breathing are at higher risk of respiratory depression.

Children and adolescents:

Behavioural changes, hostility and emotional lability are more commonly reported in younger patients. Close monitoring is recommended.

Pregnant or breastfeeding women:

Gabapentin should be used in pregnancy only if the potential benefit justifies the risk. It passes into breast milk. Specialist advice should be sought.