Nobody knows. There's no direct correlation between the 1 death associated to JWH-018, but the person was under the influence of the substance when he unfortunately decided to take his life.
As far as I know, there's been no deaths attributed to an overdose of any synthetic cannabinoid-- the one documented case took his own life because he felt the effects were too much and too negative to handle.
Use at your own risk. Nobody knows much about these things, but the Mega Synthetic Cannabinoid thread and chainer3k would be an invaluable resource when it comes to this subject.