This research aimed to assess nonprescription medicine knowledge, first aid knowledge, and compared the knowledge scores with and without previous training among school health teachers in Phayao Province, Northern Thailand. This cross-sectional study included school health teachers from primary and secondary schools by using the quota and purposive sampling methods. The data were collected using a self-administered questionnaire with three sections: socio-demographic characteristics, nonprescription medicine assessment, and first aid assessment. The data were gathered and analyzed using descriptive and independent sample t-test analysis. A total of 152 school health teachers participated in this study, with a response rate of 94.4%. The mean age was 41.5 ± 10.6 years, and 92.8% of the teachers were females. The majority (97.4%) of the teachers did not have a degree related to health sciences. 73.0% of the teachers had no previous nonprescription medicine training, while 59.2% had no previous first aid training. Only 15.8% of teachers were knowledgeable in nonprescription medicine, and only a few (0.7%) were knowledgeable in first aid. In addition, higher knowledge scores of nonprescription medicines (P-value < 0.001, 95% CI = 1.71 – 3.69) and first aid (P-value < 0.001, 95% CI = 2.83 – 4.19) were statistically significant among the teachers who had received previous training. This highlights the importance of training teachers about the use of nonprescription medicines and first aid management so they can respond appropriately to first aid needs in schools and could be incorporated into a support policy and integrated into the teachers’ training curriculum.