Many studies have shown an increased prevalence of right-to-left shunts in migraineurs. The underlying mechanisms of association between the right-to-left shunts (RLS) and migraine may concern with platelet aggregation and paradoxical embolization. In the present study, migraineurs without taking prophylactic medicines regularly will be given only clopidogrel 75 mg daily for three months. Migraineurs who are unresponsive to at least two commonly applied preventive medications, clopidogrel 75 mg daily will be added to his/her existing prophylactic migraine regimen for three months. Patients will be followed up at the third month and the sixth month for headache frequency, attack duration of headache, migraine disability assessment scale（MIDAS）scores, headache impact test（HIT-6）scores and visual analogue scale（VAS）scores. Investigators will evaluate the changes of these measurements between baseline and the follow-up time. The aim of the study is to evaluate the prophylactic effectiveness of clopidogrel for migraineurs with RLS.