The NYPD Commissioner says he would have some concerns about the legalization of recreational marijuana, after Mayor de Blasio says he supports the idea.

Following a promotion ceremony in Lower Manhattan, Commissioner James O'Neill expressed his concerns, including underage use and the sale of weed on the black market.

"One of my most serious concerns is driving while impaired. Everybody knows there is not an instantaneous test for that now and it requires drug recognition and we're going to need some time get more people up to speed on this," NYPD Commissioner James O’Neill said.

O'Neill also said the department would need to re-train or replace the dogs currently in the K-9 unit.

The mayor released a detailed report Thursday on how the city will handle recreational marijuana, if the state legalizes it.

It says smoking pot should be banned in public places and sales should be limited to people who are 21 or older, but that anyone younger caught using it, should face only civil penalties.

As for people who have a criminal record from prior marijuana arrests, the mayor called for their records to be erased.