Cannabis can be detected in hair for up to 90 days after the last use. In some cases, detection can extend to 6 months or longer, including for heavy, long-term users or when body hair is tested. The exact window depends on how much and how often cannabis was used, individual metabolism, hair length, and the type of hair sample collected.

How Hair Follicle Tests Detect Cannabis

Hair follicle tests look for THC and its metabolites, such as THC-COOH, that are deposited in the hair shaft from the bloodstream. As hair grows, these compounds become trapped inside the hair. A standard test uses about 1.5 inches (3.8 centimeters) of hair taken from the crown of the head. Since head hair grows about 0.5 inches (1.25 centimeters) per month, this length represents roughly 90 days of growth. Body hair grows slower, so a sample from the body may reflect a longer period of use.

Factors That Affect How Long Cannabis Stays in Hair

Several factors change how long cannabis remains detectable in hair:

  • Frequency and amount of use: Heavy daily use leads to more THC accumulation and a longer detection window than occasional use.
  • Metabolism: People with faster metabolisms may clear THC and its metabolites more quickly, but hair testing can still detect past use for months.
  • Hair color and type: Some research suggests that dark hair binds more THC than light hair, but this effect is not consistent across all studies.
  • Hair treatments: Bleaching, dyeing, and relaxing can damage the hair shaft and reduce the amount of THC detected, but they do not guarantee a negative result.
  • Hair length: If the hair sample is longer than 1.5 inches, the lab can test older segments of hair, which may extend the detection window to 6 months or more.
  • Body hair versus head hair: Body hair grows slower, so it may retain THC for a longer period. Some employers collect body hair if head hair is not available.

How Long After Use Can Hair Tests Detect Cannabis?

After a single use, THC and its metabolites usually appear in hair within 7 to 10 days. Once in the hair, they remain detectable until that portion of hair grows out and is cut. For a standard 1.5-inch head hair sample, the detection window is about 90 days from the last use. For heavy users, the window can be longer. If a longer hair sample is collected, a test can go back 6 months or even 12 months. However, most workplace and legal tests use the 90-day standard.

Hair Testing vs. Other Drug Tests

Hair testing has the longest detection window of all common drug tests. Here is how it compares:

  • Urine: Detects cannabis for 3 to 30 days after last use, depending on frequency.
  • Blood: Detects THC for a few hours to a few days.
  • Saliva: Detects cannabis for 24 to 48 hours.
  • Hair: Detects cannabis for up to 90 days, and sometimes longer.

Hair tests are less common in routine workplace screening due to cost and the potential for external contamination. However, they are used in some pre-employment, legal, and treatment settings.

What to Know If You Face a Hair Test

Hair drug tests are difficult to beat. Shaving your head will not help, because a collector can use body hair instead. Some detox shampoos claim to remove THC from hair, but scientific evidence for their effectiveness is lacking. In rare cases, external contamination from cannabis smoke or handling can cause a positive result, but most labs use washing procedures to reduce this risk. If you are subject to a hair test, the most reliable approach is to understand the detection window and plan accordingly.

Key Takeaways

  • Cannabis stays in hair for up to 90 days in most cases.
  • Heavy use, longer hair samples, and body hair can extend the detection window to 6 months or more.
  • Hair tests have a longer detection window than urine, blood, or saliva tests.
  • Hair treatments may reduce detection but do not guarantee a negative result.
  • Second-hand smoke is unlikely to cause a positive hair test.