Abstract only 206 Background: Methadone (M) is frequently used for severe cancer pain using the parenteral and oral route. The most commonly used dose ratio (DR) parenteral: oral is 1:2. However, methadone is highly bioavailable and a lower ratio might result in similar analgesia with less toxicity. The main objective of this RCT is to compare success and side effects with 2 ratios of parenteral to oral M: 1:2 vs 1:1.2 in hospitalized patients with cancer pain. Methods: Inpatients with cancer pain well controlled with parenteral M requiring rotation to the oral route. Double blind RCT. Outcomes included pain intensity (BPI), opioid toxicity (CTCAE), and M dose. Success was defined as good pain control with no toxicity at 72hs. Results: 39/44 randomized patients were evaluable (89%): 21 in DR 1:2 and 18 in DR 1:1.2. 71% male, median age 65. No significant difference between DR1:2 and DR1:1.2 in frequency of neuropathic pain (64 Vs 68%), Papscore A/B (100 Vs 91%), CAGE + (23 Vs 18%). Median M dose pre/post was 24.5mg±13.5 y 49 mg±27.3 for DR 1:2, Vs 23.3mg±9.4 (p: NS) y 28mg±11.3 (p < 0.01) for DR 1:1.2. The DR1:2 group developed more cumulative toxicity at dasy 1, 2 and 3 (p < 0.015, p < 0.006 y p < 0.001 respectively). Pain intensity pre/ post was: 1.58±1.3 and 0.87±1.0, ns for DR 1:2, Vs 1.13±0.7 (p:NS) and 1.07±0.9 (p:NS) for DR 1:1.2. Success was observed in 12 pts in DR1:2 Vs 18 in DR 1:1.2, p < 0.001. Side effects related to M were observed in 33/46 pts in DR 1:2 (mainly neurotoxicity symptoms) Vs 1/6 in DR 1:1.2. Conclusions: DR 1:1.2 when changing from parenteral to oral M resulted in lower toxicity and no difference in analgesia. More conservative dose adjustment during M route change should be considered. Granted by Spanish Ministry of Health EC10-133. EUDRACT Number: 2010-024092-39. Clinical trial information: 2010-024092-39.