D	Search	Hits
#1	MeSH descriptor: [Migraine Disorders] explode all trees	1832
#2	MeSH descriptor: [Post-Exposure Prophylaxis] explode all trees	39
#3	MeSH descriptor: [Pre-Exposure Prophylaxis] explode all trees	28
#4	headach*	210148
#5	MeSH descriptor: [Valproic Acid] explode all trees	734
#6	placebo	180379
#7	migraine frequenc*	907
#8	migraine attac*	1398
#9	#1 or #2 or #3 or #4	21633
#10	#7 or #8	1849
#11	#5 and #6 and #9 and #10 in Cochrane Reviews (Reviews and Protocols) and Other Reviews	16

MeSH, Medical Subject Headings.

^*A symbol used to broaden the search by obtaining variations of words that have the same letters prior to it.