The short answer

Half-life is the time it takes for your body to clear half of a substance. For THC, that first phase runs about 1 to 3 hours, then the terminal phase stretches to roughly 20 to 30 hours. Heavy daily users can push that terminal number past 5 days. CBD behaves differently, sitting around 18 to 32 hours after a single dose, and longer with repeated high doses.

Here is the part people miss. Half-life tells you when levels drop by half, not when they hit zero. After five half-lives, roughly 97 percent is gone in theory, and fat tissue keeps releasing traces long after that. That gap is why someone can feel sober for weeks and still fail a urine screen.

What half-life actually means

Drug clearance follows exponential decay. Each half-life removes a fixed fraction of whatever is left, so the curve flattens instead of dropping in a straight line. A compound with a 30 hour half-life is down to 3 percent of peak after about five days under ideal lab conditions.

Real bodies are not ideal lab conditions. Blood flow, liver enzymes, body composition, and how often you dose all bend the curve. Two people can take the same amount and land in completely different places 72 hours later.

THC half-life: two phases

THC moves through your system in stages, and each stage has its own number.

  • Distribution phase: roughly 1 to 3 hours as THC leaves blood and settles into fat, brain, and other tissue.
  • Terminal elimination phase: about 20 to 30 hours in occasional users, longer in people who use daily.
  • Metabolites: THC breaks down into 11-OH-THC, which is active, and THC-COOH, which is not. THC-COOH is what most urine tests look for, and its half-life runs 3 to 13 days.

That last point explains a lot. The molecule getting measured in a drug test is not the molecule that made you feel high, and it lingers far longer.

CBD half-life

CBD clears more slowly than THC in most studies. Single-dose research puts the half-life somewhere between 18 and 32 hours, and the numbers shift with how you take it. Oral oil absorbs slowly and peaks late. Inhaled CBD spikes fast and drops fast.

Prescription CBD tells an interesting story here. The FDA label for Epidiolex lists a half-life of 56 to 61 hours in healthy adults after repeated dosing. That is a big jump from the single-dose figures, and it comes from steady-state accumulation plus how CBD interacts with liver enzymes like CYP3A4 and CYP2C19.

That enzyme interaction matters for anyone stacking supplements or prescriptions. CBD can slow the breakdown of other drugs, which effectively stretches their half-life too.

What shifts your numbers

  • Frequency of use. This is the biggest factor by far. Daily use saturates fat stores and slows everything down.
  • Body fat percentage. THC is fat soluble. More fat means more storage and a longer, slower release.
  • Route of administration. Inhaled hits peak fast and clears faster. Edibles and oils take longer to absorb and linger longer.
  • Liver health and genetics. Enzyme activity varies a lot between people, and some gene variants slow metabolism noticeably.
  • Age and hydration. Older adults and dehydrated users tend to clear compounds more slowly.
  • Other medications. Enzyme inhibitors and inducers can push clearance either direction.
  • Dose size. Higher doses take proportionally longer to fall below any threshold.

How long cannabis stays detectable

Half-life is the theory. Detection windows are the practice, and they vary by test type.

  • Urine: 1 to 3 days after a single use, 3 to 7 days for moderate use, and 10 to 30 days or more for chronic heavy use.
  • Blood: THC itself is usually measurable for 1 to 2 days, while carboxy-THC can show up for a week or longer in frequent users.
  • Saliva: typically 1 to 3 days, shorter than urine in most cases.
  • Hair: up to 90 days, since it captures exposure over the growth window rather than current levels.

Why daily users see longer tails

Chronic use creates a reservoir effect. THC and its metabolites park in fat tissue, then leach back into blood at a slow, steady rate. That trickle keeps urine levels above detection thresholds long after the last dose and long after any psychoactive effect has worn off.

I look at half-life numbers as a range, not a promise. Someone using once a week may clear in days. Someone using multiple times a day for years can test positive for over a month. Same plant, very different math.

The bottom line

THC has a short initial half-life and a long tail. CBD clears slowly and gets slower with repeated dosing. Neither number tells you exactly when your system will be clean, because individual factors swing the results so widely.

This is general information, not medical or legal advice. If a drug test or a medication interaction is on the line, talk to a clinician who can look at your specific situation.