Background: Bone metastasis usually causes severe pain in cancer patients. The solely use of opioid may be inadequate or exert intolerable side effects. Experts suggested the use of analgesic adjuvants. However, gabapentinoids; anticonvulsants used to treat neuropathic pain, are frequently added to the pain control regimen. There is no evidence that gabapentin is useful in controlling pain due to bone metastasis. Objectives: To assess the analgesic efficacy and side effects of gabapentin 900 mg/day compared with placebo as add-on therapy to opioids in patients with metastatic bone pain.

Methods: The study is a randomized, double-blinded, fix - dose, placebo-controlled crossover trial. Thirty - two patients, who were on stable dose of opioids for the treatment of metastatic bone pain, were randomly allocated into 2 treatment groups, 16 each. Patients in each group received 2 treatment periods with 4 - day washout in between; 10 days for gabapentin or matching placebo and reciprocal. The subjects took either gabapentin 900 mg/day or placebo in identical pattern. Overall time was 24 days during the double-blind treatment period. The subjects were contacted by phone on day 2, 4, 8, 16, 18 and 22 following a dose adjustment to assess compliance and tolerability. The subjects were followed up at clinic on day 1, 15 and 25. Outcome comparisons were; average pain score reduction on day 1 and day 8 of each period, percentage of patients who have average pain score reduction ≥ 3, amount of rescue analgesic and side effects.

Results: Of the 32 patients enrolled, 30 patients received gabapentin and 26 patients received placebo. Analysis of covariance (ANCOVA) on the intention-to-treat basis showed no significant difference in average pain score reduction between gabapentin and placebo group; 2.77 ± 2.10 vs 2.46 ± 1.25, (p = 0.533). Percentage of patients who had average pain score reduction ≥ 3 during follow-up days were not different (53.23 ± 2.81 vs 47.44 ± 3.53), p = 0.586. Rescue medication for breakthrough pain was not different (p = 0.607). Finally, incidence of side effects; sedation, dizziness, nausea, vomiting and constipation, were not different.

Conclusions: Adding gabapentin to opioid analgesic did not accentuate efficacy of opioids in relieving pain in metastatic bone pain patient.