Study	Reason for exclusion
Abse 1956	Not active placebo (see Appendix 2).
Eriksson 2019	Described as a bad active placebo. Included in a sensitivity analysis.
Gracely 1987	Described as active placebo post hoc (after results). Included in a sensitivity analysis.
Ibera 2018	Not active placebo (see Appendix 2).
Max 1988	Potential active placebo implied post hoc (after results). Included in a sensitivity analysis.
Yamagata 1973	Not active placebo (see Appendix 2).
Yuge 1973	Not active placebo (see Appendix 2).