Many patients who require migraine preventive treatment have not been able to tolerate or have not responded to multiple previous preventive medications. We aimed to assess the safety and efficacy of galcanezumab, an antibody to calcitonin gene-related peptide, in patients with migraine who had not benefited from preventive medications from two to four categories.CONQUER was a multicentre, randomised, double-blind, placebo-controlled, phase 3b trial done at 64 sites (hospitals, clinics, or research centres) in 12 countries (Belgium, Canada, Czech Republic, France, Germany, Hungary, Japan, the Netherlands, South Korea, Spain, the UK, and the USA). Patients were 18-75 years of age, with episodic or chronic migraine, with migraine onset before the age of 50 years, who had a documented failure of preventive medications from two to four drug categories in the past 10 years owing to lack of efficacy or tolerability, or both. Patients were randomised 1:1 to receive subcutaneous placebo or galcanezumab 120 mg per month (with a 240 mg loading dose administered as two 120 mg injections) for 3 months. For masking purposes, patients receiving placebo also received two injections during the first dosing visit. Randomisation was done by a computer-generated random sequence by means of an interactive web-response system stratified by country and migraine frequency (low frequency episodic migraine, four to fewer than eight migraine headache days per month; high frequency episodic migraine, eight to 14 migraine headache days per month and fewer than 15 headache days per month; chronic migraine, at least eight migraine headache days per month and at least 15 headache days per month). The primary endpoint was the overall mean change from baseline in number of monthly migraine headache days during the 3-month treatment period in all patients who were randomly assigned and received at least one dose of study drug. This trial is registered with ClinicalTrials.gov, NCT03559257, and is now completed.Between Sept 10, 2018, and March 21, 2019, 462 participants with episodic (269 [58%]) or chronic (193 [42%]) migraine were randomly assigned and received at least one injection with placebo (n=230) or galcanezumab (n=232). Galcanezumab-treated patients had significantly greater reduction in migraine headache days versus placebo across months 1-3. The galcanezumab group had on average 4·1 fewer monthly migraine headache days compared with baseline (13·4), while the placebo group had on average 1·0 fewer than at baseline (13·0; between-group difference -3·1 [95% CI -3·9 to -2·3]; p<0·0001; effect size=0·72). Types and number of treatment-emergent adverse events were similar between galcanezumab and placebo. Treatment-emergent adverse events were reported in 122 (53%) of 230 patients in the placebo group and 119 (51%) of 232 patients in the galcanezumab group. There were four serious adverse events during the study, two (1%) reported in the placebo group and two (1%) reported in the galcanezumab group.Galcanezumab was superior to placebo in the preventive treatment of migraine and was safe and well tolerated in patients for whom multiple previous standard-of-care preventive treatments had failed. Galcanezumab might represent an important treatment option for patients who have not benefited from or tolerated previous standard-of-care treatments.Eli Lilly.