After reading the article entitled "Comparison of subarachnoid opioid mix for cesarean section" published by Isabel Osorio et al., in January 2020; I would like to make the following observations and comments.

The number of patients studied (n =71) is surprising considering that the hospital where the research was conducted is a third-level institution and a referral center for high complexity obstetric care. During the duration of the trial (one year, from June 2028 through May 2019) most likely many patients underwent this type of surgical intervention. Is it then possible to consider a potential selection bias? Moreover, the group distribution is not clearly defined, and certainly it was not random: A (29), B (16), C (26) ^1,2.

With the regards to the use of hydromorphone, the evidence with this drug in pregnant patients is limited. As mentioned in the medication's data sheet: there are no clinical data available for gestation. However, reproductive toxicity has been found in animal trials, and the potential risk of using opioids during pregnancy in humans is unknown ³. The use of some opioids during pregnancy and delivery may reduce or affect the uterine contractility and increase the risk of neonatal respiratory depression ^4,5.