City lawmakers – including Council speaker Corey Johnson – are demanding the Health Department answer questions about its apparently sudden decision last month to ban CBD, a cannabis oil served in drinks.

“We are deeply alarmed by the opaque nature of the process by which the Department came to this seemingly abrupt policy shift,” Johnson and Councilmen Mark Levine and Robert Holden wrote in their letter to Health Commissioner Oxiris Barbot.

The letter requests the agency provide a detailed accounting of its decision-making process, an official explanation of its stance on the compound and its possible impacts on public health and explain why the decision was made without public hearings.

The letter comes a month after the Health Department dropped the hammer on sales of the substance, a move that came with little forewarning that left coffee shops and other local retailers scrambling.

The compound, CBD, has grown in popularity in recent years because of its reputation as a stress and anxiety reducer. It is derived from the marijuana plant but does not get users high, experts say.

“There’s been no explanation about why such a drastic step needed to be taken now,” Levine told The Post. “We really want to have a full public airing of this issue.”

The Health Department said it was abiding by federal government guidelines.

“The U.S. Food and Drug Administration has advised that it is unlawful to add cannabidiol (CBD) to food or drink,” said department spokeswoman Stephanie Buhle.