how does THC affect the body

What Is Marijuana? How THC Affects the Brain and Body

For employers running a workplace drug screening program, marijuana is the substance that generates the most confusion, the most policy debate, and the most legal complexity. Before any of that becomes useful to act on, it helps to step back and answer the underlying questions: what is marijuana, and what does THC actually do in the body? This article covers the foundational biology. A companion article in this series covers how long THC lasts in the body and what impairment looks like in the workplace.

According to the National Institute on Drug Abuse, marijuana is the most frequently used federally illicit drug in the United States, with an estimated 43.5 million past-year users age 12 or older. Its workplace relevance is not abstract. NIDA-cited research has documented that workers testing positive for marijuana have shown 55% more industrial accidents, 85% more injuries, and 75% greater absenteeism than workers who test negative. The science of why sits in how THC interacts with a system in the human body that most people have never heard of.

What is marijuana?

Marijuana is the dried flowering tops, leaves, stems, and seeds of the cannabis plant — primarily Cannabis sativa, with related species Cannabis indica and Cannabis ruderalis. The plant contains more than 100 naturally occurring chemical compounds called cannabinoids, plus terpenes and flavonoids.

According to peer-reviewed research published by the National Institutes of Health, the two most-studied cannabinoids are:

  • Delta-9-tetrahydrocannabinol (THC) — the primary psychoactive compound. THC is what produces the high and what causes impairment.
  • Cannabidiol (CBD) — a non-intoxicating cannabinoid found in some forms of cannabis. NIH research describes CBD as generally non-impairing at low and moderate doses, though some sedation effects have been reported at higher doses.

Marijuana is consumed in several forms — smoked (joints, pipes, water pipes), vaporized, ingested as edibles, taken as oils or tinctures, or applied as topicals. The route of consumption substantially changes how fast THC enters the body, how strong the effect is, and how long it lasts.

According to NIH-cited pharmacokinetic research, when cannabis is inhaled, THC and its metabolites enter the bloodstream rapidly through the lungs and reach peak levels within 6 to 10 minutes. The bioavailability of inhaled THC is roughly 10% to 35%. Oral ingestion through edibles takes substantially longer — onset is typically 30 minutes to 2 hours, peak effects can occur 2 to 4 hours after ingestion, and effects tend to last longer because THC is metabolized differently when it passes through the liver before reaching the bloodstream.

How does THC affect the body?

To understand how THC affects a worker on the job, it helps to start with a system that already exists in everyone's body: the endocannabinoid system.

The endocannabinoid system

The endocannabinoid system is a communication network in the brain and body. According to NIDA, the system is named after the marijuana plant because researchers discovered it while investigating how THC works. The body produces its own cannabinoid-like molecules — called endocannabinoids — that act as neurotransmitters, sending chemical messages between nerve cells. The two main endogenous cannabinoids are anandamide and 2-arachidonoylglycerol (2-AG).

These endocannabinoids attach to specific receptors on nerve cells, called CB1 and CB2 receptors:

  • CB1 receptors are concentrated in the brain — particularly in regions that control movement, memory, thinking, concentration, coordination, sensory perception, and time perception. According to peer-reviewed neuroscience research published in PMC, CB1 receptors mediate the psychoactive effects of THC.
  • CB2 receptors are found primarily on immune cells and play roles in immune function, inflammation, and pain modulation.

The endocannabinoid system normally fine-tunes communication between nerve cells. It plays a critical role in regulating mood, pleasure, memory, learning, motor control, and stress response.

What happens when THC enters the system

THC's chemical structure closely resembles the body's own endocannabinoids — close enough that THC binds to and activates CB1 receptors directly. This is the core mechanism. According to NIDA, when a person uses cannabis, THC overwhelms the endocannabinoid system, quickly attaching to cannabinoid receptors throughout the brain and body, interfering with the ability of natural cannabinoids to do their normal job of fine-tuning communication between neurons.

The downstream effects are well-documented in peer-reviewed neuroscience research. THC binding at CB1 receptors:

  • Disrupts the hippocampus, the brain region responsible for forming new memories — which is why short-term memory is impaired during use
  • Disrupts the orbitofrontal cortex, affecting attention, decision-making, and the ability to shift focus between tasks
  • Disrupts the cerebellum and basal ganglia, brain regions that regulate balance, posture, coordination, and reaction time
  • Activates the brain's reward system, releasing dopamine at higher levels than typical natural rewards — the mechanism that contributes to marijuana's addictive properties

Why the dose-response relationship matters

According to NIH-cited clinical research, THC produces a dose-dependent disruption of cognitive and psychomotor functions. Higher doses produce stronger and longer-lasting effects. The potency of cannabis products available today is also substantially higher than products from earlier decades — modern flower commonly contains 15% to 25% THC, while concentrates and edibles can deliver much higher doses in single servings. This matters for workplace impairment because today's typical doses produce stronger acute effects than the doses studied in older research literature.

What this means for workplace screening programs

For employers, the key takeaway from the biology is that THC is the active compound that causes impairment, and CB1 receptor activation is the mechanism. Everything else — detection windows, testing methods, cutoff levels, and state employment law — is downstream of these basic facts.

The most common practical questions that follow from the biology include:

  • How long does THC stay in the body, and how does that compare to how long impairment actually lasts?
  • What does cannabis impairment look like in a worker, and how can supervisors document it?
  • Which testing methods detect the active compound, and which detect inactive metabolites?
  • What does this mean for choosing between urine and oral fluid testing?

These topics are covered in detail in the supporting articles in this series.

Final takeaway

Marijuana is the dried plant material of Cannabis sativa, containing more than 100 cannabinoids, of which delta-9 THC is the primary psychoactive compound. THC produces its effects by binding to CB1 receptors in the endocannabinoid system — a network of nerve cell receptors that the body uses for its own internal cannabinoid signaling. By overwhelming this system, THC disrupts normal communication between nerve cells in brain regions that control memory, attention, motor coordination, judgment, and reaction time. The dose-dependent nature of these effects, combined with the higher potency of modern cannabis products, is the foundation for understanding why marijuana matters for workplace safety and why testing programs are designed the way they are.

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Sources

  • National Institute on Drug Abuse, "Cannabis (Marijuana) — How does marijuana produce its effects?" (current edition)
  • National Institute on Drug Abuse, Cannabis (Marijuana) Research Report
  • National Institutes of Health / PMC, "Mechanisms of Action and Pharmacokinetics of Cannabis"
  • National Institutes of Health / PMC, "Molecular Mechanism and Cannabinoid Pharmacology"
  • National Institutes of Health / PMC, "Turning Over a New Leaf: Cannabinoid and Endocannabinoid Modulation of Immune Function"
  • National Institutes of Health / PMC, "The neuropsychopharmacology of cannabis: A review of human imaging studies"

Disclosure: My BIO TESTS® sells rapid urine and oral fluid drug screening tests in three regulatory categories: CLIA-Waived FDA 510(k)–cleared, Employer & Insurance Use Only, and Forensic Use Only. Selection of the appropriate category depends on your testing program and use case. Positive results from any rapid screening test are presumptive and should be confirmed by an HHS-certified laboratory and reviewed by a qualified Medical Review Officer (MRO). This content is educational and is not medical, legal, or scientific advice for any specific testing program. Consult qualified professional counsel before making policy or testing decisions for your workforce.

Last updated: May 2026

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