Quality of evidence profile for the assessed outcomes, adapted from GRADE [25].

Notes:
All studies had observational design.
Publication bias could not be objectively assessed due to the small numbers of studies.
Imprecision could not be assessed since adequate meta-analysis calculation could not be performed.
RR: relative risk.
NE: Non-estimable.
−We could not access the item for this outcome.
*Flawed measurement of outcome, as the study did not aimed to report the incidence of adverse reaction in pregnant women.
†Some studies did not report the incidence of adverse reaction; the data was obtained with the authors.
‡Heterogeneous results across studies were observed.
§Different population (patients were not pregnant women and did not have syphilis) and intervention. In one study [95] other types of penicillin, besides benzathine, may have been used.