The Government has brought forward the start date of legislation that makes it an offence to drive while under the influence of drugs.
The law was to have come into effect on 1 December but will now take effect from 1 November.
The legislation allows police to carry out a roadside "impairment" test which, if failed by the driver, may be followed by a blood test.
Transport Minister Steven Joyce says the November start chimes with the new road-user rule banning the use of cellphones while driving.
He says the scope of the legislation will also be widened to include benzodiazepines, which are sedatives and anti-anxiety medications.
But it won't catch out legitimate users of the medications, he says.