Appendix 1. MEDLINE Search Strategy (PubMed format)

1      Acetaminophen [mh]
2      acetaminophen [tiab] or paracetamol [tiab]
3      #1 or #2
4      Headache [mh] or Headache Disorders [mh]
5      headach* [tiab] or cephalgi* [tiab] or cephalalgi* [tiab]
6      #4 or #5
7      #3 and #6
8      randomized controlled trial [pt]
9      controlled clinical trial [pt]
10    randomized [tiab]
11    placebo [tiab]
12    drug therapy [sh]
13    randomly [tiab]
14    trial [tiab]
15    groups [tiab]
16    #8 or #9 or #10 or #11 or #12 or #13 or #14 or #15
17    animals [mh] not humans[mh]
18    #16 not #17
19    #7 and #18