Drug tests can show if someone has been using illegal substances in the past. For instance, employers will sometimes ask new employees to take drug tests to ensure that they aren't using illegal substances. This is especially true for dangerous occupations or those – such as being a police officer – where the use of a firearm is required.

But do these tests actually demonstrate any level of impairment? If a police officer pulls someone over on suspicion of drunk driving, that officer can give them a breath test. This shows their blood alcohol concentration. If it is over 0.08%, then the court can presume that that person is impaired by alcohol. Do drug tests essentially work the same way, but for substances other than alcohol?

The difference between use and impairment

The short answer is no, drug tests generally do not demonstrate impairment. There's a major difference between using these substances in the past and being impaired at the time.

After all, some drugs can show up in hair tests for the next 90 days. Drugs may be found in urine tests for seven days, and those who are chronic users could even have positive results after a week. Even oral fluid tests may demonstrate evidence of drugs for the next 48 hours.

But someone who was driving a vehicle may have used illegal substances 48 hours or even 90 days earlier, and they are certainly no longer impaired. They are not under the influence at all because the drug's effects simply don't last that long. This is why a drug test cannot be used the same way that breath tests are used with alcohol.

Are you facing criminal accusations and do you have questions about the tests that were used? Be sure you understand all of your legal defense options.

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