Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: “A randomisation program was used to allocate volunteers into the two groups” Comment: adequately done
Allocation concealment (selection bias)	Low risk	Quote: “The 1 mg, 5 mg and placebo pills were of the same colour and size” Comment: adequately done
Similar baseline characteristics	Unclear risk
Similar baseline outcome measurement	Unclear risk
Blinding of participants and personnel (performance bias)	Low risk	Quote:“neither the authors nor the health staff or volunteers were aware of the subject's dose category until the study was completed. ”Comment: adequately done
Blinding of outcome assessment (detection bias)	Low risk	Quote: “neither the authors nor the health staff or volunteers were aware of the subject's dose category until the study was completed.” Comment: adequately done
Incomplete outcome data (attrition bias)	Low risk	Quote: Follow‐up rates at the end of the study were fifty‐eight out of seventy (82.9%) for the 5 mg once weekly dosage group and fifty out of seventy(71.4%) for the 1 mg daily dosage group. Comment: difference is <30% Quote: “Eleven volunteers in the 5 mg/week group dropped out from the study because of the following reasons: four resigned from work, three became pregnant, one had low B12 and three did not want to continue in the trial. Among the women in the 1 mg/d group, eight dropped out because they resigned their positions, two were fired, five became pregnant, one had low B12, two had burning and discomfort, and two did not give any specific reason.” “Furthermore, in the majority of cases the cause for dropping out was not linked to the trial per se.” Comment: reason for not continuing were provided
Prevention of knowledge of allocated intervention	Unclear risk
Protection against contamination	Unclear risk
Selective reporting (reporting bias)	Unclear risk	Comment: insufficient information to permit judgement, and no protocol was found.
Other bias	High risk	Quote: “(i) the two groups had different time lags between last pill taken and blood draw, which could have biased the group receiving the daily dosage to observed higher levels in serum and red blood cell folate; (ii) we were not able to observe the long‐term effect of the two dosage regimens as the intervention and measurement of folic acid levels spanned only 12 weeks; and (iii) we did not assess the un metabolized folic acid in serum nor the dietary intake of the participants. Recent research has demonstrated that, with higher dosages of folic acid, one can expect amounts of un metabolized folic acid to be found in blood. Our study was not designed to evaluate un metabolized folic acid.” Comment: other biases were present