Carbamazepine (Tegretol)

An anticonvulsant for superior oblique myokymia and trigeminal neuralgia; carries an FDA Boxed Warning and requires HLA-B*1502 screening in patients of Asian ancestry.

Drug Class: Anticonvulsants

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Carbamazepine (brand name Tegretol) is an anticonvulsant medication also used for nerve-related pain and certain movement disorders. In neuro-ophthalmology, it's effective for superior oblique myokymia.

FDA Boxed Warnings - carbamazepine

The FDA label for carbamazepine carries two boxed warnings:

Serious dermatologic reactions - Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Risk is substantially higher in patients with the HLA-B*1502 allele (more common in several East Asian, Southeast Asian, and South Asian populations). Prescribing guidance recommends considering HLA-B*1502 testing before starting in patients with ancestry from higher-prevalence populations, and avoiding carbamazepine in carriers unless the benefit clearly outweighs the risk.
Aplastic anemia and agranulocytosis - rare, but life-threatening. Baseline and ongoing CBC monitoring is required.

A separate label warning notes increased risk of SJS, TEN, DRESS, and AGEP in patients known to carry the HLA-A*3101 allele (more common in patients of European, Japanese, Korean, and Latin American ancestry). The FDA label does not mandate routine pre-treatment HLA-A*3101 screening; if a patient is already known to be positive, the prescriber should weigh the benefits and risks before starting.

Key Takeaways

Anticonvulsant used for superior oblique myokymia, trigeminal neuralgia, epilepsy
FDA boxed warnings for serious skin reactions and aplastic anemia
HLA-B*1502 screening is recommended for patients of Asian ancestry before starting; HLA-A*3101 is associated with cutaneous reactions in other populations and is one of several factors a prescriber may weigh, though routine pre-treatment testing is not mandated by the FDA label
Many drug interactions - carbamazepine is a strong CYP3A4 inducer
Start low, go slow, and monitor blood counts and liver function

How It Works

Carbamazepine stabilizes nerve membranes by blocking sodium channels, reducing abnormal nerve firing. This helps control conditions caused by hyperexcitable nerves.

Common Uses

Superior oblique myokymia
Trigeminal neuralgia
Epilepsy
Bipolar disorder

Dosing

Start low (100-200mg daily)
Increase gradually
Typical therapeutic dose: 200-400mg twice daily
Extended-release formulation available

Side Effects

Common

Dizziness
Drowsiness
Nausea
Unsteadiness

Serious (Require Monitoring)

Blood count changes (aplastic anemia, agranulocytosis-rare but serious)
Liver problems
Severe skin reactions (especially in certain genetic backgrounds)
Low sodium

Important Monitoring

Blood monitoring required: Carbamazepine can affect blood counts and liver function. Regular blood tests are needed, especially early in treatment.

Before Starting

Check CBC and liver function
Consider HLA-B*1502 genetic testing (for certain populations-risk of severe skin reactions)
Review drug interactions

Drug Interactions

Carbamazepine is a strong inducer of CYP3A4 and several other liver enzymes. It can lower the level (and effectiveness) of many co-administered drugs, including:

Hormonal contraceptives (birth control pills, patches, rings, implants) - backup contraception is needed
Warfarin and direct oral anticoagulants
Other antiepileptic drugs (lamotrigine, valproate, phenytoin, perampanel)
Many SSRIs and antipsychotics
Macrolide antibiotics (erythromycin, clarithromycin) - can dramatically raise carbamazepine levels and cause toxicity
Grapefruit juice - raises carbamazepine levels
Direct-acting antivirals and many oral chemotherapy agents

Carbamazepine also undergoes autoinduction: blood levels often drop after the first few weeks because the body is induced to clear it faster, requiring a dose adjustment. Always inform every clinician and pharmacist about every medication and supplement you take.

Frequently Asked Questions

Why do I need blood tests?

Carbamazepine can rarely cause dangerous drops in blood cell counts or liver problems. Regular monitoring catches these early.

Can I drink alcohol?

Alcohol can worsen side effects like drowsiness and dizziness. Use caution or avoid alcohol.

What if I need surgery?

Inform your surgical team-carbamazepine affects how other medications work.

References

For current U.S. drug labeling, contraindications, boxed warnings, pregnancy/lactation language, and formulation-specific dosing, check the official label databases and your prescriber's instructions.

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