Last verified: April 2026

CBD as a CYP450 Inhibitor — The IC50 Data

The most comprehensive characterization of CBD's drug interaction potential comes from Bansal et al. (2020, Drug Metabolism and Disposition), who determined unbound IC50 values (IC50,u) for CBD across the major CYP450 isoforms using human liver microsomes. The unbound IC50 — the concentration of free (non-protein-bound) drug required to inhibit enzyme activity by 50% — is the pharmacologically relevant measure because only unbound drug accesses enzyme active sites.

The Bansal data, ranked by inhibitory potency:

• CYP2B6: IC50,u = 0.05 µM — extremely potent inhibition
• CYP2C9: IC50,u = 0.17 µM — potent inhibition
• CYP2C19: IC50,u = 0.30 µM — potent inhibition
• CYP3A4: IC50,u = 0.38 µM — moderate-to-potent inhibition
• CYP1A2: IC50,u = 0.45 µM — moderate inhibition
• CYP2D6: IC50,u = 0.95 µM — moderate inhibition

These IC50,u values are clinically achievable at the CBD doses used in medical settings (300–1,400 mg/day). Even consumer-grade CBD products at 50–100 mg/day may produce sufficient hepatic concentrations to inhibit the most sensitive isoforms (2B6, 2C9). The breadth of inhibition — six major CYP enzymes — is what makes CBD's interaction potential exceptional.

Mechanism-Based (Time-Dependent) Inhibition

For three isoforms — CYP1A2, CYP2C19, and CYP3A4 — CBD produces time-dependent inhibition (TDI), also called mechanism-based inhibition. Unlike reversible competitive inhibition, which dissipates as CBD concentrations fall, TDI involves CBD forming a reactive intermediate that covalently modifies and permanently inactivates the enzyme. Recovery requires synthesis of new enzyme protein, a process taking 24–72 hours.

The clinical consequence: the inhibitory effect of CBD on these three enzymes persists after CBD is eliminated. A patient who discontinues CBD may still have suppressed CYP1A2, CYP2C19, and CYP3A4 activity for 1–3 days. This has implications for medication adjustment timing and for the common but pharmacologically naive practice of skipping CBD doses on the day of other medication use.

Phase II Inhibition — UGT Enzymes

Beyond CYP450 inhibition, CBD also inhibits UGT1A9 and UGT2B7, phase II conjugation enzymes that glucuronidate drugs for renal excretion. This adds a second layer of drug interaction risk: even drugs that escape CYP-mediated interactions may have their clearance impaired at the glucuronidation step. Substrates of UGT1A9/2B7 include certain opioids (morphine), NSAIDs (diclofenac), and statins (atorvastatin).

THC — The Clinically Insignificant Inhibitor

In sharp contrast to CBD, THC at typical recreational or medical doses (5–20 mg) does not produce clinically significant CYP450 inhibition in vivo. While in vitro studies show THC can inhibit CYP enzymes at high concentrations, the unbound plasma concentrations achieved after standard THC doses are well below inhibitory thresholds for meaningful drug interactions.

This distinction is critical: cannabis drug interaction risk is overwhelmingly a CBD phenomenon, not a THC phenomenon. Patients using THC-dominant products without CBD have substantially lower pharmacokinetic interaction risk. The clinical implication is that interaction screening should focus on CBD exposure specifically, not cannabis use generally.

High-Risk Medication Combinations

Warfarin — The Hemorrhage Risk

Grayson et al. (2018) reported the most alarming documented interaction: a patient on stable warfarin therapy initiated CBD oil and experienced an INR increase from 2.0 to 6.86 — more than triple the therapeutic target, placing the patient at serious hemorrhage risk. The mechanism is CBD inhibition of CYP2C9, the primary warfarin metabolizer (the same enzyme that handles THC).

Smythe et al. (2023) compiled 7 published cases of CBD-warfarin interactions, with hemorrhage documented in at least one case. The INR elevations were substantial (typically 2–3x baseline) and occurred within days to weeks of CBD initiation. This interaction is classified as high-risk: warfarin has a narrow therapeutic index, and supratherapeutic anticoagulation can be fatal.

The patient's INR increased from a therapeutic 2.0 to a dangerous 6.86 after initiating CBD oil, consistent with CYP2C9 inhibition of warfarin metabolism.

Grayson et al., Journal of Internal Medicine 2018

Clobazam — The Epidiolex Signal

The clobazam interaction is the most thoroughly characterized CBD drug interaction thanks to the Epidiolex clinical trial program. CBD inhibits CYP2C19-mediated metabolism of clobazam, increasing plasma concentrations of the active metabolite N-desmethylclobazam by approximately 3-fold. In Epidiolex trials, somnolence/sedation occurred in 46% of patients receiving CBD plus clobazam compared to 16% receiving clobazam alone — a nearly 3-fold increase in sedation directly attributable to the pharmacokinetic interaction.

The FDA-approved Epidiolex label mandates clobazam dose reduction when co-administered with CBD, making this the first formally regulated cannabis drug interaction.

Tacrolimus — The Transplant Emergency

So et al. (2025, Phase I study) conducted the first controlled pharmacokinetic study of CBD-tacrolimus interaction in 12 healthy volunteers. The results were dramatic: CBD increased tacrolimus Cmax by 4.2-fold and AUC (total exposure) by 3.1-fold. The mechanism is CBD inhibition of CYP3A4, the primary tacrolimus metabolizer.

Tacrolimus is a calcineurin inhibitor used to prevent organ transplant rejection. Its therapeutic index is extremely narrow — subtherapeutic levels risk organ rejection, supratherapeutic levels cause nephrotoxicity and neurotoxicity. A 4.2-fold increase in peak concentration is potentially organ-threatening. This interaction represents an absolute contraindication for unsupervised CBD use in transplant patients.

Valproate — The Hepatotoxicity Signal

Co-administration of CBD and valproic acid (Depakote/Epilim) produces a hepatotoxicity signal that exceeds what either drug produces alone. In Epidiolex trials, ALT elevations exceeding 3 times the upper limit of normal occurred in 21% of patients receiving CBD plus valproate compared to 3% receiving CBD without valproate. The mechanism likely involves combined stress on hepatic conjugation pathways and mitochondrial toxicity, though the precise interaction is not fully characterized.

Other High-Concern Combinations

Statins: CYP3A4-metabolized statins (atorvastatin, simvastatin, lovastatin) are at risk for increased plasma concentrations via CBD inhibition. Elevated statin levels increase the risk of rhabdomyolysis — a potentially life-threatening muscle breakdown syndrome. Rosuvastatin (primarily renally cleared) and pravastatin (not CYP3A4-dependent) are lower-risk alternatives.

SSRIs: Several selective serotonin reuptake inhibitors are metabolized by CYP2C19 (citalopram, escitalopram) or CYP2D6 (fluoxetine, paroxetine). CBD inhibition of these enzymes could elevate SSRI levels, increasing risk of serotonin syndrome when combined with other serotonergic agents. The clinical significance at consumer CBD doses (10–50 mg) is less certain than at medical doses (300+ mg).

Opioids: Morphine glucuronidation via UGT2B7 may be impaired by CBD, potentially increasing morphine exposure. Codeine activation to morphine via CYP2D6 could be inhibited, paradoxically reducing codeine efficacy. Fentanyl metabolism via CYP3A4 could be slowed, increasing overdose risk. Each opioid-CBD interaction has a different mechanism and direction, requiring drug-specific assessment.

The Grapefruit Analogy — Why It Undersells the Risk

Clinicians frequently compare CBD's CYP inhibition to grapefruit juice — a familiar reference point for patients. The analogy is directionally correct but substantially understates CBD's interaction potential. Grapefruit juice primarily inhibits intestinal CYP3A4, affecting first-pass metabolism of orally administered drugs. CBD inhibits 6 or more CYP enzymes (2B6, 2C9, 2C19, 3A4, 1A2, 2D6) plus UGT enzymes, affecting both intestinal and hepatic metabolism of drugs administered by any route.

The breadth of CBD's inhibitory profile means it interacts with a far wider range of medications than grapefruit, and the time-dependent inactivation of CYP1A2, 2C19, and 3A4 means the effects persist longer. A patient who is told "CBD interacts with drugs like grapefruit does" may reasonably conclude the risk is modest and manageable. The pharmacological reality is more serious.

The CANN-DIR Database

The most comprehensive resource for cannabis drug interaction assessment is CANN-DIR (Cannabis Drug Interaction Resource), developed by Vrana and Kocis at Penn State University and accessible at cann-dir.psu.edu. The database catalogs known and predicted interactions between cannabinoids and conventional medications, providing mechanism-level detail and clinical significance ratings.

CANN-DIR is currently the closest approximation to a comprehensive cannabis drug interaction checker available to clinicians and pharmacists. Its development reflects growing recognition that cannabis drug interactions are a clinical safety priority, not an academic curiosity — particularly as patient populations using both cannabinoids and conventional medications continue to expand.