Actually yes:

https://en.wikipedia.org/wiki/Synthetic_cannabinoids

They typically cause more severe side-effects than traditional marijuana. There have been a few rounds of subway ads related to synthetic marijuana.

And to follow up, with a NYC example:

http://www.nytimes.com/2016/07/13/nyregion/k2-synthetic-mari...

I used to be an organic chemist. These fake marijuana substitutes are pretty horrific: unscrupulous vendors read the scientific literature for newly developed cannabinoid receptor activating chemicals. These are then sold, without proper testing in humans usually.

The side effect profile of these compounds is worse than natural cannabis, with high incidence of psychotic symptoms.

https://en.m.wikipedia.org/wiki/Synthetic_cannabinoids

Edit: i completely overlooked rpedroso's substantially identical reply, haha...

I think that just means contraband lettuce.

No it's a reference to K2 which has actually been a huge problem in NYC. A month or two ago there were like a couple dozen people hospitalized in a single morning

This refers to 'synthetic marijuana' (K2, Spice, etc.)