Antiepileptic Drug Interactions
Older antiepileptic drugs are potent enzyme inducers that affect contraceptives, anticoagulants, and immunosuppressants. Newer agents have fewer but still relevant interactions.
Enzyme-Inducing AEDs
The classic trio of carbamazepine, phenytoin, and phenobarbital are powerful inducers of CYP3A4, CYP2C9, CYP2C19, UGT enzymes, and P-glycoprotein. Their effects are broad and clinically devastating when not anticipated.
Oral Contraceptive Failure
Enzyme-inducing AEDs accelerate ethinylestradiol and progestogen metabolism, causing breakthrough bleeding and unintended pregnancy. Standard combined oral contraceptives are unreliable. Options for women on inducing AEDs:
- Depot medroxyprogesterone acetate (Depo-Provera) — unaffected
- Levonorgestrel IUD — local action, unaffected
- Copper IUD — non-hormonal
- High-dose OCP (50 mcg ethinylestradiol) — if hormonal method preferred
Emergency contraception with levonorgestrel is also less effective; copper IUD is preferred.
Warfarin and DOACs
Carbamazepine and phenytoin induce warfarin metabolism, requiring dose increases of 50-100%. DOACs (apixaban, rivarexaban) are CYP3A4 and P-gp substrates; their levels are substantially reduced by enzyme-inducing AEDs, and co-administration is generally avoided.
Chemotherapy
Many oncology drugs are CYP3A4 substrates. Enzyme-inducing AEDs reduce the efficacy of irinotecan, cyclophosphamide active metabolites, tyrosine kinase inhibitors, and vinca alkaloids. NCCN guidelines recommend switching to non-inducing AEDs (levetiracetam, lacosamide) before starting chemotherapy.
Enzyme-Inhibiting AEDs
Valproic Acid
Valproate inhibits UGT enzymes and epoxide hydrolase. Key interactions:
- Lamotrigine — valproate doubles lamotrigine half-life from 25 to 60 hours, increasing rash and Stevens-Johnson syndrome risk. Lamotrigine dose must be halved when adding valproate.
- Carbamazepine epoxide — valproate inhibits epoxide hydrolase, raising the toxic 10,11-epoxide metabolite while carbamazepine induces valproate metabolism. Bidirectional interaction.
- Lorazepam — valproate inhibits glucuronidation, increasing lorazepam levels
Phenytoin (Dual Role)
Phenytoin is both an inducer and a saturable-metabolism drug following zero-order kinetics near therapeutic levels. Small dose changes or added CYP2C9 inhibitors (fluconazole, amiodarone) can cause disproportionate toxicity (nystagmus, ataxia, lethargy).
Low-Interaction AEDs
The newer AEDs were specifically developed to minimize drug interactions:
| Drug | CYP Interaction | Notes |
|---|---|---|
| Levetiracetam | None | Renally excreted; safest in polypharmacy |
| Lacosamide | Minimal | Mild CYP2C19 substrate |
| Brivaracetam | Mild CYP2C19 | Less interaction than carbamazepine |
| Gabapentin/pregabalin | None | Renally excreted |
| Zonisamide | Minimal | Weak CYP3A4 substrate |
Key Takeaways
- Carbamazepine, phenytoin, and phenobarbital induce multiple CYP and UGT enzymes broadly
- Oral contraceptive failure is a major risk; IUDs are preferred for women on inducing AEDs
- Switch to levetiracetam or lacosamide before chemotherapy to avoid reduced drug efficacy
- Valproate doubles lamotrigine levels; dose adjustment and slow titration are essential
- Newer AEDs (levetiracetam, gabapentin, lacosamide) have minimal drug interaction potential