PSNI officers are saving £1,000 a time by not pursuing drug driving charges when alcohol has been taken but no serious injury or death caused, the force said.
Investigators are pressing for excess alcohol charges where suspects have provided a sample over the limit rather than lab testing for further substances offences.
This is because a toxicology analysis involves significant extra cost and generally does not produce a more severe court sentence.
However, campaigners insisted the public need to understand the serious impact of drugs on driving and the Brake road safety charity said officers should be adequately resourced.
Spokesman Jack Kushner said: “Every day we see the devastating consequences of crashes caused by drug drivers and people need to understand that these substances will seriously affect their ability to drive safely.”
The PSNI said it was not accurate to suggest officers did not test for the presence of substances. The force has a significant number of trained officers who can conduct preliminary impairment tests at the roadside or in stations.
A spokesman added: “Where PSNI detect a drug driver we still have to prove that they were unfit to drive and support these observations with a toxicology report.
“In our experience both the PPS prosecutors and the courts have a better grasp of the excess alcohol offence where there is a technically derived reading that supports the police case.
“The cost to the public purse is approximately £1,000 for each toxicology report carried out.
“In those cases where the driver has already provided an alcohol sample which is over the limit and no serious injuries or death have been sustained as a result of this driving, then police will simply report for the excess alcohol offence.
“In the PSNI experience the courts do not generally regard the unfit cases, be they alcohol or drugs with any greater degree of severity than the simple excess offence. Neither do the sentences handed down markedly differ.”
PSNI chief constable George Hamilton has previously said funding pressure this year requires a 10% cut to non-staff budgets.
In 2015, 72 drivers or riders were killed or seriously injured due to being impaired by alcohol or drugs.
Mr Kushner added: “An estimated 200 lives are lost each year in the UK as a result of drug-driving, and we urge the government to keep investing in roads policing and treat it as a priority.”