Police say drug drivers are using synthetic cannabis to dodge roadside drug tests, which cannot detect the deadly substance.

Synthetic cannabis is illegal and police have said it is a battle keeping up with the synthetic drug industry.

“Synthetic drug use is increasing an is becoming more of an issue for us on the road,” Inspector Martin Boorman told 9NEWS.

Workplaces and roadside drug tests cannot identify the presence of the substance.

“The difficulty is there’s so many different of types of them and they have different chemical compounds,” Insp. Boorman said.

“It’s driven by greed, by people wanting to make money and by making money they’re causing misery.”

Synthetic cannabis is often sold at suburban adult shops under brand names such as Kronic or Spice, and contains a mixture of chemicals, solvents and herbs.

Synthetic cannabis has been linked to several deaths around Australia, including that of Melbourne man Daniel George, who died three days short of his daughter’s first birthday in late 2014.

“Two puffs of a particular product and within hours, it ended his life,” Mr George’s sister Jenny Wilson said.