Read the decree document here (Spanish)

There are a number of reports on the development.

Leafly write

First reported by Jon Hiltz at Marijuana.com, the decree arrived after months of speculation. Peña Nieto was once a vehement opponent of cannabis legalization, but has since called for a re-examination of global drug policy after a nationwide public debate on legalization in early 2016. “So far, the solutions [to control drugs and crime] implemented by the international community have been frankly insufficient,” Peña Nieto told the 2016 United Nations General Assembly Special Sessions in April 2016. “We must move beyond prohibition to effective prevention.”

More at  https://www.leafly.com/news/politics/mexico-legalizes-medical-marijuana

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Mexico Legalizes MMJ; Regulations Expected in 6 months