A Study of Galcanezumab (LY2951742) in Participants 12 to 17 Years of Age With Chronic Migraine

Inclusion Criteria: * Have a diagnosis of chronic migraine as defined by the IHS ICHD-3 guidelines (1.3 according to ICHD-3 \[2018\]), that is, a headache occurring on 15 or more days per month for at least the last 3 months, which has the features of migraine headache on at least 8 days per month. Exclusion Criteria: * Participants who are taking, or are expected to take, therapeutic antibodies during the course of the study (adalimumab, infliximab, trastuzumab, bevacizumab, etc.). Prior use of therapeutic antibodies is allowed if that use was more than 12 months prior to baseline, except for antibodies to CGRP or its receptor, which are not allowed at any time prior to study entry. * Known hypersensitivity monoclonal antibodies or other therapeutic proteins, or to galcanezumab or its excipients. * Current use or prior exposure to galcanezumab, another CGRP antibody, or CGRP receptor antibody, including those who have previously completed or withdrawn from this study or any other study investigating a CGRP antibody. Participant must also not have prior oral CGRP antagonist use within 30 days prior to baseline. * History of IHS ICHD-3 diagnosis of new daily persistent headache, cluster headache or migraine subtypes including hemiplegic (sporadic or familial) migraine and migraine with brainstem aura (previously basilar-type migraine). * History of significant head or neck injury within 6 months prior to screening; or traumatic head injury at any time that is associated with significant change in the quality or frequency of their headaches, including new onset of migraine following traumatic head injury. * Participants with a known history of intracranial tumors or developmental malformations including Chiari malformations.