How Do the Police Test for Drugs in a DUI Stop?

In California and the rest of the nation, it’s illegal to drive under the influence of alcohol or drugs, and this includes lawfully prescribed medications. In light of that, how do the police test people for drugs or alcohol, or both? Read on as we explain this in detail.

Generally, the DUI investigation begins when a law enforcement officer suspects that a driver is under the influence of something, whether it’s drugs or alcohol. An officer can become suspicious of a driver under the following types circumstances:

  • The officer observes possible signs of impairment during a DUI checkpoint. For example, the driver has an odor of alcohol on their breath, or their eyes are bloodshot, or they are slurring their speech.
  • The officer observes a suspect driving erratically, which may indicate impairment.
  • During a routine traffic stop, an officer observes signs that a driver may be under the influence of drugs or alcohol.
  • After an accident, the at-fault driver is unconscious but the police want to have the driver tested for drugs or alcohol to see if the accident could be DUI-related.
  • A concerned citizen called 911, reporting a possible drunk driver so the cops track the person down on the roadway.

When someone is suspected of DUI, usually the first thing the officer will do is ask the driver to submit to a series of field sobriety tests, which are divided attention tests that are conducted roadside. These tests are very difficult to pass, even for sober people!

After the driver performs poorly on the field sobriety tests, next the officer will ask the driver to submit to a chemical test in the form of a blood, breath or urine test. Of the three, blood tests are considered to be the most accurate, though they aren’t infallible.

Note: Field sobriety tests are not legally required and there is no penalty for refusing. However, under California’s implied consent law, if a driver fails to complete or refuses to submit to a chemical test (blood, breath or urine), their driver license will automatically be suspended for one year, even if they were not under the influence!


In DUI cases, blood and breath tests are used to test for the presence of drugs or alcohol, with urine tests being rarely used because they are less accurate. When it comes to testing for drugs, blood tests are the most reliable and are therefore used in the vast majority of DUI of drug cases.

Contact our Orange County DUI defense lawyers if you’re facing drug or alcohol-related DUI charges!

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