Routine blood work doesn’t test for THC, and for most people cannabis causes no large or reliable change in common results. Heavy or frequent use is linked to small shifts in a few markers, such as white blood cells, hemoglobin, insulin and liver enzymes. Cannabis can also raise blood levels of some prescription drugs. A standard checkup panel looks at your blood cells, organs and cholesterol, and cannabis isn’t on the list unless someone orders a drug screen on purpose.1TestWell. Does Weed (THC) Show Up on a Blood Test?
Will Routine Blood Work Show That You Smoke Weed
Your order may list a CBC (complete blood count, a count of red cells, white cells and platelets) or a CMP (comprehensive metabolic panel, which checks blood sugar, kidneys, liver and salts). Neither of these, nor a cholesterol, thyroid or hormone test, measures or reports THC.2TestWell. Does Weed (THC) Show Up on a Blood Test? A drug screen is a separate test that a doctor or employer has to request specifically.3TestWell. Does Weed (THC) Show Up on a Blood Test?
To tell whether one is on your order, look for words like “drug screen,” “toxicology” or “cannabinoids” on the lab slip or patient portal. Hospital emergency visits, surgery prep and pain-management clinics order them more often than routine checkups do. You can also ask the front desk or the doctor what is being drawn before the needle goes in. Employer tests are usually urine, not a blood draw at your doctor’s office.
A blood THC test usually looks for THC or its breakdown products, which stay in the body for different lengths of time, so a positive result is not a precise record of when you last used. A positive screen shows exposure, not impairment or a use disorder, and many screens are confirmed with a second, more specific lab test before anyone acts on them.
Drug screens for THC also react to similar compounds, including delta-8 THC, which triggers the test even more strongly than regular THC does.4PubMed. Determination of Cross-Reactivity of Contemporary Cannabinoids with THC Direct Immunoassay (ELISA) in Whole Blood Delta-10 THC and THC-O cause much weaker reactions, so they may or may not show up.5PubMed. Determination of Cross-Reactivity of Contemporary Cannabinoids with THC Direct Immunoassay (ELISA) in Whole Blood If you use hemp-derived delta-8 products, assume a cannabis drug screen can come back positive.
Weed and Your Blood Count
In a large US national health survey, only heavy cannabis users had higher white blood cell counts than people who had never used.6PubMed Central. Total and differential white blood cell count in cannabis users: results from the cross-sectional National Health and Nutrition Examination Survey, 2005–2016 Occasional use didn’t change the count. Across many studies combined, cannabis users had somewhat higher counts of white cells overall and of two white cell types your report may list, neutrophils and monocytes.7PubMed Central. The association between hematological markers of inflammation and chronic cannabis use: a systematic review and meta-analysis of observational studies The rise was under 1,000 cells per microliter on average, while a normal count spans roughly 4,500 to 11,000.8PubMed Central. The association between hematological markers of inflammation and chronic cannabis use: a systematic review and meta-analysis of observational studies Most users’ counts would still read as normal.
A slightly high white count has many more common causes than cannabis. Infection, stress, tobacco smoking and some medicines raise white cells far more. If your count is borderline high, mentioning heavy smoking helps the doctor weigh it, and a doctor usually rechecks a mildly high count rather than acting on one result.
People who currently use marijuana had hemoglobin about 0.1 g/dL higher than never users, a tiny amount against a normal range of about 12 to 17.5.9PubMed. Marijuana Use and Hemoglobin Concentrations in NHANES 2009-2018: Implications for Subclinical Hypoxemia Smoke lowers oxygen slightly, and the body may make a bit more hemoglobin to compensate.10PubMed. Marijuana Use and Hemoglobin Concentrations in NHANES 2009-2018: Implications for Subclinical Hypoxemia Former users showed no clear difference, which suggests the effect fades after stopping.11PubMed. Marijuana Use and Hemoglobin Concentrations in NHANES 2009-2018: Implications for Subclinical Hypoxemia
Platelet findings come mostly from lab dishes and animals. In lab dishes, THC made human platelets more ready to clot.12PubMed. The procoagulatory effects of delta-9-tetrahydrocannabinol in human platelets Monkeys given a daily THC edible had platelets that clumped less, the opposite direction.13PubMed Central. Chronic edible dosing of Δ9-tetrahydrocannabinol (THC) in nonhuman primates reduces systemic platelet activity and function There is no clear effect on a routine platelet count to expect.
Blood Sugar, Insulin and Cholesterol
Chronic smokers had fasting glucose and A1C (a three-month average of blood sugar) similar to non-smokers.14PubMed Central. Metabolic Effects of Chronic Cannabis Smoking Fasting insulin tends to run lower. Current users had about 16% lower fasting insulin and better insulin-resistance scores in a US national survey.15PubMed. The impact of marijuana use on glucose, insulin, and insulin resistance among US adults The difference was strongest in people with obesity, where even under four uses a month was linked to insulin about half as high.16PubMed. Lifetime marijuana use in relation to insulin resistance in lean, overweight, and obese US adults This is an association, and it doesn’t mean cannabis treats diabetes. Nobody should skip prescribed diabetes treatment based on these findings.
Cholesterol results are inconsistent. One study found lower total and LDL cholesterol in medical marijuana users.17Arteriosclerosis, Thrombosis, and Vascular Biology. Abstract 577: Medical Marijuana Use: Effect on Lipid Metabolism and Atherosclerosis Reviews find results vary widely between studies and allow no firm conclusion.18PubMed. Cannabis effects on lipoproteins
For fasting tests, what you eat matters more than the weed itself. Fasting glucose, insulin and triglycerides are the numbers most sensitive to a recent meal, which is why labs ask you to fast. An edible the night before a fasting draw counts as food and breaks the fast.19TestWell. Does Weed (THC) Show Up on a Blood Test? Stick to water from the fasting start time the lab gives, often 8 to 12 hours. If you broke the fast, tell the lab so they can note it or reschedule.
Liver and Kidney Results
ALT and AST are liver enzymes on your metabolic panel that rise when liver cells are irritated. ALT sits mostly in liver cells, while AST is also found in muscle and other tissues, so AST can rise for reasons that have nothing to do with the liver.
In young adult smokers in Nigeria, AST and ALT were higher than in non-smokers, and 8 in 10 had AST above the normal limit.20Journal of Medical Laboratory Science (or University of Benin Institutional Repository). POSSIBLE EFFECT OF MARIJUANA CONSUMPTION ON LIVER FUNCTION AMONG YOUNG ADULTS IN BENIN CITY AST was roughly double in cannabis users in another Nigerian study.21PubMed. Impact of cannabis, benzodiazepines, and methamphetamine use on inflammatory and hepatic biomarkers in Benin City, Nigeria: a case-control study These studies are small and local, and mixed drug use and other habits may play a part.
A mild rise does not diagnose liver disease by itself; it tells the doctor to look for a cause. Alcohol, fatty liver, some medicines and hard exercise are more common causes. Strenuous exercise in the day or two before a draw can raise AST, because muscle breakdown releases it into the blood. If you lifted heavily or ran a race before your test, mention it.
If your liver enzymes come back high, tell your doctor about cannabis and alcohol use so they can sort out causes. A doctor will often repeat the test after a few weeks, and cutting back on heavy use before the recheck is reasonable.
Kidneys look unaffected. In nearly 14,000 US adults, cannabis use made no meaningful difference to creatinine or eGFR, the kidney-filtering estimate on most reports.22PubMed. Marijuana Use and Renal Function Among US Adults
Thyroid, Hormone and Vitamin Tests
TSH, thyroid-stimulating hormone, is the main thyroid number on most reports. It rises when the thyroid is underactive, so a high value is the usual first sign the doctor looks for. Recent users were less likely to have a high TSH in one study.23PubMed. Effect of Marijuana Use on Thyroid Function and Autoimmunity Another study found no effect on thyroid-stimulating hormone or thyroid hormones.24PubMed Central. Clinical Investigation on the Impact of Cannabis Abuse on Thyroid Hormones and Associated Psychiatric Manifestations in the Male Population With results that split, a thyroid result outside range shouldn’t be put down to cannabis.
Chronic use showed no effect on testosterone, reproductive hormones, prolactin or cortisol in men or women.25PubMed. Effects of chronic marijuana use on testosterone, luteinizing hormone, follicle stimulating hormone, prolactin and cortisol in men and women Frequent users did have lower prolactin than non-users in another study.26PubMed Central. The effects of cannabinoids on serum cortisol and prolactin in humans Smoking can raise cortisol for about an hour, though not consistently, and heavy users show a weaker response.27PubMed Central. Impact of Acute and Chronic Cannabis Use on Stress Response Regulation: Challenging the Belief That Cannabis Is an Effective Method for Coping28PubMed Central. Marijuana Use and Hypothalamic-Pituitary-Adrenal Axis Functioning in Humans So cortisol drawn soon after smoking may read differently.
Cortisol and testosterone are often drawn in the morning because levels change through the day. Avoid smoking before a morning hormone draw to keep the result clean, and mention heavy use if a hormone result surprises your doctor.
Nutrient levels, albumin and hemoglobin were similar between users and non-users in a US survey.29Public Health Nutrition. Dietary intake and nutritional status of US adult marijuana users: results from the Third National Health and Nutrition Examination Survey Among people with severe substance use problems, frequent users had slightly lower vitamin D.30PubMed Central. Vitamin D status and associations with substance use patterns among people with severe substance use disorders in Western Norway
Medicines Your Doctor Monitors
THC and CBD block liver enzymes that clear many medicines.31PubMed. Cannabinoid Interactions with Cytochrome P450 Drug Metabolism: a Full-Spectrum Characterization Many drugs are cleared this way, so when cannabis compounds slow those enzymes, the same pill can leave more drug in your blood than before. In lab studies, nearly all cannabis compounds blocked an enzyme that helps clear warfarin, at levels that can occur in users.32PubMed. Cannabinoid Interactions with Cytochrome P450 Drug Metabolism: a Full-Spectrum Characterization
Higher blood levels can mean more side effects such as drowsiness or, with blood thinners, a greater bleeding risk, even though you take the dose as prescribed. The effect depends on the dose and form of cannabis, which is why the prescriber needs the specifics.
Rising CBD doses changed blood levels of several seizure medicines, though they stayed in safe ranges.33PubMed Central. Contemplating cannabis? The complex relationship between cannabinoids and hepatic metabolism resulting in the potential for drug-drug interactions CBD at 200 to 800 mg a day raised citalopram levels in people taking citalopram or escitalopram.34PubMed Central. Contemplating cannabis? The complex relationship between cannabinoids and hepatic metabolism resulting in the potential for drug-drug interactions These findings concerned CBD at doses used in studies, so low-dose products may have smaller effects, though that is not guaranteed.
Warfarin is adjusted using a blood clotting test called INR, which you may see on your results, taken regularly to keep the dose in a narrow range. If cannabis slows warfarin’s clearance, INR can climb, so starting, stopping or changing how much you use is a reason to tell the clinic that manages your dose.
Worth mentioning to your prescriber: blood thinners like warfarin, seizure medicines, lithium and some antidepressants with level checks, and transplant drugs that have narrow safe ranges. Tell them about any CBD oil, edible or smoking, including the amount.
Pregnancy Screening Blood Tests
Second-trimester screening measures markers such as AFP in the mother’s blood to estimate the chance of certain conditions in the baby. These screens estimate risk and are not diagnoses, so an abnormal result usually leads to follow-up testing.
Cannabis didn’t change AFP.35PubMed Central. Effects of alcohol, cigarettes, methamphetamine and marijuana exposure during pregnancy on maternal serum alpha-fetoprotein levels at 20-24 weeks’ gestation Pregnancy hormone levels were also normal in marijuana users.36PubMed. Pregnancy hormone concentrations in marijuana users Experts say there is little data and more research is needed.37American Journal of Clinical Pathology. Tobacco and Cannabis Use During Pregnancy: Analysis From a Maternal Quadruple Screening Program
A normal screen doesn’t mean cannabis is safe for the baby. The studies looked only at screening markers, not at the baby’s development or health. Tell your prenatal provider about use, since some clinics also test urine for drugs, and ask about stopping.
Should You Stop Before Your Test
Most changes linked to cannabis come from heavy, ongoing use, not a single session. Skipping use the night and morning before mainly protects the fast and cortisol timing, and lets you arrive clear-headed for the draw.
A day off will not clear a THC drug screen in a regular user, since THC from frequent use can linger longer than a day. If a screen worries you, ask whether one is included rather than relying on a short break.
You don’t need to stop for weeks before most routine tests, because the shifts are small. Cutting back makes more sense if a recheck shows something unexpected, such as higher liver enzymes, and your doctor suggests it.
When you talk to your doctor, say how often and in what form you use: smoked, vaped, edible or CBD. Mention delta-8 or hemp products too, since they act like THC. Medical records are private, and doctors ask in order to interpret results, not to judge.