Dutch police may begin testing the saliva of drivers and people involved in violent crimes for drugs from July 1 when new zero-tolerance rules come into effect, the transport ministry said.

The saliva test can be used to find out if people have taken opiates, cocaine, THC (the active ingredient in marijuana) and methamphetamines and has been used in Belgium for some time.

The test identifies whether there are drugs in the system but cannot say how much or when the drug was taken, which needs to be established to find out if someone has broken road safety laws.

To establish this, follow up tests need to be done, Patrick van Vugt from test maker Dräger told BNR radio.

The test is not without its critics. Cannabis researcher Nicole Maalsté said in a website column that THC can be identified up to 14 hours after it was taken. ‘I consider it very unlikely that someone is still under the influence if they smoked week half a day ago,’ she said.

Follow up blood tests are also expensive, Maalsté said. This makes it likely that police will select who to run a saliva test on based on their appearance. ‘Shirt and tie, no drugs test, cap and sneakers, yes a drugs test,’ she wrote. ‘I can see it happening.’