Laced weed is so rare that the entire concept may as well be categorized as a myth.
I have seen articles about cheap Hungarian weed sprayed with Chinese synthetic "canabis" being sold as spice. Apparently it's stronger and more addictive.
But I must admit I have not double checked it.
Just found a bit here: https://en.wikipedia.org/wiki/Synthetic_cannabinoids
Nor does it particularly make sense to me... why not just spray on delta THC-8 or 9, the whole point of synthetic cannabinoids is they don't show up on drug tests.