NK,   In the case of hemiplegic Migraine, verapamil (a calcium channel blocker) is often the preventive medication first tried because there's more documentation of it being effective for that type of Migraine.   Other than that, it's not really a matter of differential diagnosis. There are several issues that come into play when a doctor is choosing a Migraine preventive medication:   1) The type of doctor and how much experience he has treating Migraine disease. Unless the doctor is an actual Migraine and headache specialist, their experience treating Migraine is probably not much. Additionally, these doctors simply don't have the time to stay up-to-date on the treatments for all conditions. As a result, they may tend to prescribe the medication that's being talked about most at the time. In the case of Migraine, it's antiseizure medications, particularly Topamax since Topamax is the medication most recently approved by the FDA for the prevention of Migraine.   2) Other medical conditions the patient takes medications for. Sometimes, the Migraineur will be taking medications for other conditions, and some of the medications for the other conditions can also be used for Migraine prevention. In such cases, it's fairly common for the doctor treating the Migraines to work with the doctor treating the other conditions to try to find one medication that can treat both conditions. Common examples of this are: Hypertension or heart disease and Migraine -- beta blockers such as Inderal and several other types of medications that treat hypertension and heart disease are also excellent Migraine preventives for some people. Depression and Migraine -- several antidepressants are also good Migraine preventives.   3) The patient's reactions to and experience with other medications they've tried.   The best case scenario is when the doctor and patient discuss preventive medication options and make treatment choices as a team.   Hope this helps! Teri