Drug Crimes Center · Procedure Guide

Why you should not take hair and urine test results at face value

In drug cases, few pieces of evidence are as powerful as test results. But each type of test covers a different range, and none tells you the date or number of uses. You need to sort out what the results prove and what they do not.

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Key Points

  • The rapid reagent test used at the scene is a first-stage screening; conclusions come from forensic analysis at the National Forensic Service (NFS) or similar institutions.
  • Generally, urine is used to confirm use within the last few days, and hair, in proportion to its length, use up to several months ago (this varies by drug and by person).
  • Hair analysis can show only during which period drug use occurred; it cannot identify exact dates or the number of uses.
  • You can refuse a test requested on a consent basis, but with a court warrant samples can be taken by force.

1. Features of each test method

  • Rapid urine reagent test · A screening test that gives results within minutes at the scene. The possibility of cross-reactivity with other substances must be considered.
  • Forensic urine analysis · The NFS or a similar institution analyzes the substances and their concentrations. We check the substances detected, the time of collection, and how the sample was stored.
  • Hair analysis · The hair is divided into segments to check for drug use. The site and length of collection, the results for each segment, and any dyeing or bleaching are important.
  • Blood test · Used to confirm very recent use, mainly in accidents or emergencies. The time of collection is key.

2. How long drugs can be detected

The detection period varies considerably depending on which drug was used, how much, how many times, and the individual's constitution. Urine is commonly understood to cover the last few days and hair the months corresponding to its length, but this is only a tendency, not the answer in any individual case. You need to read the figures and segments in the forensic report directly.

3. Points that can be challenged

  • Cross-reactivity · A rapid test can come out positive for ingredients in cold medicine or certain prescription drugs, so the actual substance is confirmed separately in forensic analysis.
  • Timing and number of uses · Investigative agencies sometimes rely on hair results to set a broad period and number of uses. We point to the limitation that the analysis cannot pinpoint dates.
  • Collection and storage procedures · We check who collected the sample and when, how it was sealed and transported, and whether it could have been mixed up with other samples.
  • External contamination and passive exposure · It is possible to argue that you inhaled smoke or touched a contaminated object, but whether this is persuasive must be weighed against the concentrations and segment results.

4. Can you refuse a test?

You may decline a request to cooperate without a warrant. However, once a warrant is issued, samples are taken by force, and the fact that you refused can add to suspicion. Before testing, organize a list of medications you have recently taken and your medical history.

Frequently Asked Questions

Can I avoid a hair test by shaving my head or dyeing my hair?
If there is no hair on your head, body hair from other areas can be collected, and signs that you deliberately obstructed the test work against you. Rather than trying to avoid the test, it is better to interpret the results correctly and challenge them.
My hair test came out positive, but I actually used only once.
Hair analysis shows only that drug use occurred during the relevant segment; it does not establish the number of uses. Explaining that it was a single use, combined with other evidence, can prevent the number of uses from being found too broadly.

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Written on the basis of Korean statutes in force (National Law Information Center, Ministry of Government Legislation) and court precedents as of October 20, 2026. Results may differ with amendments to the law or the facts of each case. This page is general information, not legal advice.

Written and reviewed by Attorney Dongwoo Shin — Ph.D. in Law; certified criminal law specialist and certified divorce law specialist registered with the Korean Bar Association. He handles each case personally from consultation to judgment. About the attorney →
Last updated October 20, 2026

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