# Cannabidiol Inhibits Phenytoin Clearance: Two Clinical Cases > **Author:** Benjamin Caplan, MD (ABFM Board Certified, MA Lic. #240562, NPI: 1962722637) > **Publisher:** CED Clinic (https://cedclinic.com) > **Canonical URL:** https://cedclinic.com/cbd-phenytoin-interaction-clinical-impact-monitoring/ > **Published:** 2026-04-14T01:02:28-04:00 | **Last Modified / Clinically Audited:** 2026-09-28T10:22:30-04:00 > **Clinical Summary:** Explore the CBD phenytoin interaction and its clinical impact on seizure management, highlighting the need for careful monitoring and dose adjustments. --- CED Clinical Relevance  #50Monitored Relevance  Early-stage or contextual signal requiring further evidence before action. 🔬 Evidence Watch  |  CED Clinic EpilepsyDrug InteractionsCbdPhenytoinCase Series Journal Seizure Study Type [Clinical Study](https://cedclinic.com/cannabis-hyperemesis-syndrome-psychiatric-risk/) Population Human participants Why This Matters This case series provides crucial real-world evidence of CBD’s ability to inhibit phenytoin clearance, a [drug interaction](https://cedclinic.com/glp-1-addiction-risk-reduction-benefits/) with significant clinical implications for seizure management. While phenytoin’s induction of CBD metabolism was known, this bidirectional interaction demonstrates CBD can meaningfully alter phenytoin levels in both beneficial and harmful directions. Clinical Summary Two Lennox-Gastaut Syndrome patients experienced clinically significant changes in phenytoin levels after CBD initiation. The first achieved seizure freedom when CBD elevated both phenytoin and clobazam levels, but seizures returned when phenytoin was reduced despite continued clobazam elevation, suggesting phenytoin’s contribution was critical. The second patient developed phenytoin toxicity with ataxia and falls, which resolved after dose reduction. These cases confirm predicted CYP450-mediated interactions between CBD and phenytoin have meaningful clinical consequences. Dr. Caplan’s Take “I routinely monitor antiepileptic drug levels when initiating CBD in my epilepsy patients, and these cases reinforce why that’s essential practice. The bidirectional nature of this interaction means we need to be prepared for both therapeutic enhancement and toxicity.” Clinical Perspective 🧠 Clinicians should monitor phenytoin levels closely when starting or adjusting CBD in patients on phenytoin therapy. Dose adjustments of either medication may be necessary to optimize efficacy while avoiding toxicity. Patients should be counseled on signs of phenytoin toxicity including ataxia, dizziness, and coordination problems. 💬 Join the Conversation Have a question about how this applies to your situation? [Ask Dr. Caplan →](https://benjamincaplan.com/ask-dr-caplan) Want to discuss this topic with other patients and caregivers? [Join the forum discussion →](https://cedclinic.com/interact/discussion-forums/forum/cannabis-industry/) Have thoughts on this? Share it: [𝕏 Share on X](https://twitter.com/intent/tweet?text=Just%20read%20this%20on%20CEDclinic.com%3A%20Cannabidiol%20inhibits%20phenytoin%20clearance%20and%20can%20result%20in%20clinical%20changes%3A%20Two...%20%40drcaplan&url=https%3A//pubmed.ncbi.nlm.nih.gov/41764811/)[in Share on LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A//pubmed.ncbi.nlm.nih.gov/41764811/)[🦥 Share on BlueSky](https://bsky.app/intent/compose?text=Just%20read%20this%3A%20Cannabidiol%20inhibits%20phenytoin%20clearance%20and%20can%20result%20in%20clinical%20changes%3A%20Two...%20--%20via%20%40drcaplan.bsky.social%20https%3A//pubmed.ncbi.nlm.nih.gov/41764811/)[📷 Follow on Instagram](https://www.instagram.com/drbenjamincaplan/)[📝 Read more on Substack](https://substack.com/@doctorapproved)[🔔 Subscribe via RSS](https://cedclinic.com/category/cannabis-news/feed/) 📰 Source: [https://pubmed.ncbi.nlm.nih.gov/41764811/](https://pubmed.ncbi.nlm.nih.gov/41764811/) Table of Contents - [FAQ](#FAQ)[Does CBD affect phenytoin levels in patients with epilepsy?](#Does_CBD_affect_phenytoin_levels_in_patients_with_epilepsy) - [What are the signs of phenytoin toxicity when combined with CBD?](#What_are_the_signs_of_phenytoin_toxicity_when_combined_with_CBD) - [Should phenytoin doses be adjusted when starting CBD in epilepsy patients?](#Should_phenytoin_doses_be_adjusted_when_starting_CBD_in_epilepsy_patients) - [Can the CBD-phenytoin interaction improve seizure control?](#Can_the_CBD-phenytoin_interaction_improve_seizure_control) - [What monitoring is needed when combining CBD with phenytoin?](#What_monitoring_is_needed_when_combining_CBD_with_phenytoin) - [Related](#Related) ## FAQ #### Does CBD affect phenytoin levels in patients with epilepsy? Yes, CBD can significantly increase phenytoin levels by inhibiting its metabolism. This case series demonstrates that adding CBD to phenytoin therapy can lead to clinically meaningful increases in phenytoin concentrations, potentially resulting in both [therapeutic benefits](https://cedclinic.com/therapeutic-benefits-of-thc/) and toxicity. #### What are the signs of phenytoin toxicity when combined with CBD? Phenytoin toxicity can manifest as ataxia (loss of coordination) and increased fall risk, as demonstrated in the second case. These neurological symptoms resolved when the phenytoin dose was reduced after CBD was added to the regimen. #### Should phenytoin doses be adjusted when starting CBD in epilepsy patients? Yes, phenytoin doses may need to be reduced when initiating CBD therapy due to the [drug interaction](https://cedclinic.com/glp-1-addiction-risk-reduction-benefits/). Close monitoring of phenytoin levels and clinical symptoms is essential, as demonstrated by the need for dose adjustments in both cases presented. #### Can the CBD-phenytoin interaction improve seizure control? The interaction may contribute to improved seizure control through increased phenytoin levels, as seen in the first case where the patient achieved seizure freedom. However, when phenytoin was later reduced due to liver concerns, seizures returned despite continued CBD therapy, suggesting the elevated phenytoin levels were crucial for seizure control. #### What monitoring is needed when combining CBD with phenytoin? Regular monitoring of phenytoin serum levels, liver function tests, and clinical assessment for toxicity symptoms is essential. The cases highlight the importance of balancing therapeutic efficacy with potential adverse effects, requiring ongoing dose optimization and safety monitoring. 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[Learn More](https://cedclinic.com/adolescent-substance-use-suicide-risk-analysis/) ### Related --- ### Verification & Medical Governance - **Lead Clinical Author:** Benjamin Caplan, MD - **Credentials:** Board Certified in Family Medicine (ABFM), Founder & CMO CED Clinic & CED Foundation, Author of *The Doctor-Approved Cannabis Handbook* (Penguin Random House). - **Clinical Scale:** 350,000+ lifetime evaluations overseen; 81,008-patient Real-World Evidence Registry (`~/.ced_rwe_vault.db`); 125,000+ peer-reviewed study literature observatory (`http://localhost:8768/api/explorer/terminal`). - **License Verification:** Commonwealth of Massachusetts License #240562 (https://checkahealthlicense.mass.gov/). - **NPI Registry:** 1962722637 (https://npiregistry.cms.hhs.gov/provider-view/1962722637). - **Ethics & Data Privacy:** Clinical registry observations adhere strictly to HIPAA Safe Harbor de-identification (45 CFR § 164.514(b)) and criteria for secondary research exemption under 45 CFR § 46.104(d)(4).