Cannabis hyperemesis syndrome (CHS) is a condition characterized by cyclical nausea and vomiting and is associated with frequent cannabis use. Data from CDC's National Syndromic Surveillance Program were analyzed to examine trends in CHS-involved emergency department (ED) visits before and after implementation of a new, CHS-specific International Classification of Diseases, Tenth Revision, Clinical Modification diagnosis code on October 1, 2025. Monthly CHS-involved ED visits per 10,000 all-cause visits were assessed overall and by demographic characteristics. During January 2023-May 2026, a total of 199,565 ED visits involved CHS. During January 2023-September 2025, the proportion of ED visits that involved CHS remained mostly steady. In the first month after implementation of the new diagnostic code, the proportion of CHS-involved ED visits increased from 3.35 per 10,000 ED visits in September 2025 (preimplementation) to 11.26 per 10,000 ED visits in October 2025 (postimplementation). During the first 8 months after implementation of the new code (October 2025-May 2026), average monthly proportions of CHS-involved ED visits were 3.7 times as high as the monthly average during January 2023-September 2025. Higher proportions of CHS-involved ED visits were observed among persons aged 15-24 years and females, with more pronounced impacts among some demographic groups after code implementation. The abrupt, sustained increase might partly reflect improved recognition and coding of CHS rather than a true rise in incidence. These findings highlight a likely underestimated impact of CHS, suggesting the need to strengthen education about the risks associated with frequent cannabis use, expand ongoing surveillance efforts, and improve clinical recognition of CHS through continuing medical education and implementation of routine cannabis use assessment in ED settings.