migraine
"Just started this at 300mg at bedtime a few days ago, but after two days it stopped what had been a blinding 3-day headache.  The migraine is still there but currently with no pain (this undoubtedly sounds stupid to non-migraineurs, but many with migraines will know what I mean.)  I am wondering if anyone has experienced good results with this medicine but then had it suddenly stop working.  This has happened to me with so many medications, including some like Verapamil that should NOT build tolerance."