Table 5. Risk for bias assessment.

Study	Selection of participants	Confounding variables	Intervention measurement	Blinding of outcome assessment	Incomplete outcome data	Selective outcome reporting
Jalessi et al 15	–	–	+	–	+	?
Harvey et al 26	+	+	+	–	+	+
Rioja et al 19	–	–	+	–	+	+
Tam et al 23	+	–	+	–	+	+
Kim et al 24	–	–	+	–	+	?
Thompson et al 28	–	–	+	–	?	–

+, low risk of bias; –, high risk of bias; ?, unclear.