Carfentanil, one of the two fentanyl derivatives used in the Moscow theater hostage crisis was actively marketed by several Chinese chemical companies at the time. Carfentanil was not a controlled substance in China, where it was manufactured legally and sold openly over the Internet up until May 1st of 2019, when a ban on fentanyl and all fentanyl analogues went into effect. There has been controversy between the US and China over whether the Chinese ban on sales of fentanyl derivatives to the US has been effective. Fentanyl led to more than 37,000 overdose deaths in the US in 2019.