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Gabapentin Dosing Considerations
Three gabapentin products are FDA approved to treat PHN. The different formulations cannot be interchanged and each has its own dosing schedule.
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- For immediate-release gabapentin (Neurontin), dosing may be initiated with 300 mg on day 1, doubled on day 2 (300 mg twice a day), and tripled on day 3 (300 mg 3 times a day). The dose can then be titrated up as needed for pain relief to a maximum dose of 1,800 mg daily (divided into 3 daily doses). Clinical studies referenced in the package insert state that efficacy for a range of doses from 1,800 mg/day to 3,600 mg/day were observed; however, there was no additional benefit seen with doses greater than 1,800 mg/d.
- Gralise is an extended-release gabapentin formulation that also is FDA approved for PHN with a titration schedule that begins with 300 mg on day 1; 600 mg on day 2; 900 mg on days 3 to 6; 1,200 mg on days 7 to 10; 1,500 mg on days 11 to 14; and 1,800 mg on day 15 and thereafter.
- The third gabapentin formulation for PHN treatment is another extended-release product, Horizant. The starting dose is 600 mg in the morning for 3 days, increased to 600 mg twice daily on day 4 and thereafter. A daily dose of Horizant greater than 1,200 mg provided no additional benefit at the expense of side effects.
Several studies have evaluated off-label use of gabapentin in the treatment of other neuropathic pain conditions. A randomized, double-blind trial compared gabapentin to placebo in 135 patients with DPN over 8 weeks. The results showed a statistical benefit of gabapentin compared to placebo, at all end points, for pain improvement.4 The gabapentin dosing regimen used in this study was 900 mg/d for week 1; 1,800 mg/d for week 2; 2,400 mg/d for week 3; and 3,600 mg/d for week 4. All the patients were titrated up to a dose of 3,600 mg/d, regardless of efficacy at lower doses. Patients who could not tolerate this dose were titrated down to the greatest tolerable dose.
Of the 84 patients randomized to the gabapentin group, 56 (67%) were able to tolerate 3,600 mg/d.4 During the first week, gabapentin resulted in improvement in sleep interference compared to placebo. By the second week, gabapentin resulted in improvement in all pain rating scales compared to placebo. Of the 84 patients in the gabapentin group, 70 completed the study, and 7 patients withdrew due to adverse drug events (ADEs). Most ADEs reported in the gabapentin group were of mild or moderate intensity, and the most frequently reported effects were dizziness (23.8%), somnolence (22.6%), headache (10.7%), diarrhea (10.7%), confusion (8.3%), and nausea (8.3%).
A double-blind crossover study (n=40) assessed gabapentin for the treatment of DPN. The dose of gabapentin used in this trial was much lower, with patients titrated up every 3 days to a maximum dose of 900 mg/d.
The end points evaluated in this study included level of pain on a visual analog pain scale (VAS), and scores on the present pain intensity scale, the McGill pain questionnaire (MPQ), and the global assessment of pain relief. Statistical improvement between gabapentin and placebo was noted in only 1 end point, the MPQ score, with a mean reduction of 8.9 points for gabapentin compared to 2.2 points with placebo (P=0.03). No serious ADEs were noted, and the most common ADEs of gabapentin were drowsiness, fatigue, and imbalance.
The results of this study suggest that gabapentin is not effective or is only minimally effective in treating painful DPN at a dose of 900 mg/d.5
A search in the Cochrane Database of Systematic Reviews was conducted to further examine dosing regimens for neuropathic pain. In a review analyzing 37 studies for gabapentin treatment in chronic neuropathic pain, the main outcome was Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) definitions for moderate and substantial benefit in chronic pain studies.6 These were defined as follows:
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- 30% reduction in pain over baseline (moderate)
- 50% reduction in pain over baseline (substantial)
- Much or very much improved on Patient Global Impression of Change (PGIC) (moderate)
- Very much improved on PGIC (substantial)
- Gabapentin was shown to be better than placebo across all studies for IMMPACT outcomes. The review concentrated on gabapentin doses of 1,200 mg/d or greater and reported that doses at or above this threshold were reasonably effective for treatment of various neuropathic pain types. The upper threshold for maximum effective gabapentin doses ranged from 2,400 mg/d to 3,600 mg/d in the majority of studies reviewed.6 Table 1, on page 16, provides a more detailed description of the maximum gabapentin doses evaluated for different neuropathic pain types.
Usual Adult Dose for Postherpetic Neuralgia:
Initial dose: 300 mg orally on day one, 300 mg orally twice a day on day two, then 300 mg orally 3 times a day on day three.
The dose may be titrated up as needed for pain relief to a daily dose of 1800 mg. Maintenance dose: 900 to 1800 mg orally in 3 divided doses.
Efficacy was demonstrated in clinical studies over a range of 1800 mg/day to 3600 mg/day. However, no additional benefit was demonstrated from the use of doses over 1800 mg/day.
Gabapentin available under the trade name Gralise (R):
Maintenance dose: Gralise (R) should be titrated to 1800 mg orally once daily with the evening meal.
Recommended titration schedule:
Day 1: 300 mg orally with the evening meal
Day 2: 600 mg orally with the evening meal
Days 3 through 6: 900 mg orally with the evening meal
Days 7 through 10: 1200 mg orally with the evening meal
Days 11 through 14: 1500 mg orally with the evening meal
Day 15: 1800 mg orally with the evening meal
Gralise (R) is not interchangeable with other gabapentin products because of differing pharmacokinetic profiles that affect the frequency of administration.
Gabapentin enacarbil extended release tablets available under the trade name Horizant (R):
The recommended dosage is 600 mg orally twice daily. Therapy should be initiated at a dose of 600 mg orally in the morning for 3 days of therapy, then increased to 600 mg twice daily (1,200 mg/day) on day four.
Gabapentin enacarbil extended release tablets available under the trade name Horizant (R) and gabapentin are not interchangeable.
Usual Adult Dose for Restless Legs Syndrome:
Gabapentin enacarbil available under the trade name Horizant (R):
600 mg orally once daily with food at about 5 PM
Usual Pediatric Dose for Epilepsy:
Less than 3 years: Effectiveness has not been established.
Greater than or equal to 3 and less than 12 years:
Starting Dose: ranges from 10 to 15 mg/kg/day in 3 divided doses.
Effective Dose: reached by upward titration over a period of approximately 3 days. The effective dose of gabapentin in patients 5 years of age and older is 25 to 35 mg/kg/day and given in divided doses (three times a day). The effective dose in pediatric patients ages 3 and 4 years is 40 mg/kg/day and given in divided doses (three times a day). Gabapentin may be administered as the oral solution, capsule, or tablet, or using combinations of these formulations. Dosages up to 50 mg/kg/day have been well tolerated in a long term clinical study. The maximum time interval between doses should not exceed 12 hours.
Greater than 12 years:
Initial dose: 300 mg orally on day one, 300 mg orally twice a day on day two, then 300 mg orally 3 times a day on day three.
Maintenance dose: 900 to 1800 mg orally in 3 divided doses. If necessary, the dose may be increased using 300 mg or 400 mg capsules three times a day up to 1800 mg/day.
Dosages up to 2400 mg/day have been well tolerated in long term clinical studies. Doses of 3600 mg/day have also been administered to a small number of patients for a relatively short duration, and have been well tolerated.
The maximum time between doses in the three times a day schedule should not exceed 12 hours.
Gabapentin Dosage Information
Gabapentin is highly lipophilic but not bound to plasma proteins, which show linear pharmacokinetics and do not show a link with any significant protein binding or liver metabolization. It has an oral bioavailability of greater than 90%, which is independent of dose. Generally, patients achieve steady-state plasma levels within 24 to 48 hours. There is no clinically significant effect in administration with food nor on the extent of absorption or elimination. The elimination half-life of the drug is approximately 6.5 hours. Gabapentin readily crosses the blood-brain barrier. It is primarily excreted through the renal path, with no active metabolites. Dosage adjustment is necessary for patients with renal impairment. Pregabalin does not induce or inhibit CYP enzymes. Also, none of the CYP enzyme inhibitors alter its pharmacokinetics as a consequence.
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Initial treatment with gabapentin is usually started with one dose of 300 mg per day and later increases the frequency to 3 times a day and dosage up to 4800 mg per day. The recommendation is to start the first dose in the evening and then take the drug three times a day.
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Usually, the effects are apparent in the first week of treatment but sometimes take about a month for significant improvement.
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Taper the dose over more than seven days to discontinue the medication.
For Partial seizure
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300 to 1200 mg 3 times per day by mouth
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Max: 3600 mg per day
For Post-Herpetic Neuralgia
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300 to 600 mg 3 times per day by mouth
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Max: 1800 mg per day
For Neuropathic Pain
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300 to 1200 mg 3 times per day by mouth
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Max: 3600 mg per day
For Fibromyalgia
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400 to 800 mg 3 times per day by mouth
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Max: 2400 mg per day
Renal Dosing
Adjust the dose amount and frequency.
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Creatinine clearance of 30 to 60: 200 to 700 mg twice per day
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Creatinine clearance of 16 to 29: 200 to 700 mg once daily
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Creatinine clearance of 15: 100 to 300 mg once daily
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Creatinine clearance of less than 15: 125 to 350 mg as a supplement
