Pharmacology
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Central Nervous System
A 21 year old woman presents to ED with pelvic pain and vaginal bleeding. Her pregnancy test is positive. She tells you she has been taking her combined oral contraceptive pill regularly and that she has recently been started a new antiepileptic medication. Which of the following medications has most likely affected her contraception:
Answer:
Carbamazepine (and phenytoin) are enzyme-inducing drugs which may accelerate the metabolism of some drugs resulting in their reduced therapeutic effect, such as:- Corticosteroids
- Doxycycline
- Oral contraceptives
- Thyroid hormones
- Tricyclic antidepressants
- Simvastatin
- Warfarin
Carbamazepine
Pharmacology / Central Nervous System
Last Updated: 30th March 2023
Indications
- Focal and secondary generalised tonic-clonic seizures
- Primary generalised tonic-clonic seizures
- Trigeminal neuralgia
- Prophylaxis of bipolar disorder unresponsive to lithium
- Adjunct in acute alcohol withdrawal
- Diabetic neuropathy
Mechanism of Action
Carbamazepine, lamotrigine, valproate and phenytoin act by producing a use-dependent block of neuronal Na+ channels. Their anticonvulsant action is a result of their ability to prevent high-frequency repetitive activity. The drugs bind preferentially to inactivated (closed) Na+ channels, stabilising them in the inactivated state and preventing them from returning to the resting (closed) state, which they must re-enter before they can again open.
Contraindications
Carbamazepine is contraindicated in:
- People with known hypersensitivity to carbamazepine or structurally related drugs
- People with atrioventricular block (may suppress AV conduction and ventricular automaticity)
- People with a history of bone marrow depression (risk of agranulocytosis and aplastic anaemia)
- People with a history of acute porphyrias
- People taking a monoamine oxidase inhibitor (risk of serotonin syndrome)
Side Effects
Common adverse effects include nausea and vomiting, sedation, dizziness, headache, blurred vision and ataxia. These adverse effects are dose related and are most common at the start of treatment.
Other adverse effects include:
- Allergic skin reactions (and rarely, more serious dermatological conditions)
- Hyponatraemia (avoid concomitant use with diuretics)
- Leucopenia, thrombocytopenia and other blood disorders
- Hepatic impairment
Monitoring
Serum carbamazepine levels should not be routinely monitored unless toxicity is suspected. A full blood count, liver function tests, and urea and electrolyte measurements should be done before starting treatment and every 6 months during treatment.
Interactions
The plasma concentration of carbamazepine may be increased by the concomitant use of:
- Macrolide antibiotics (clarithromycin and erythromycin)
- Azole antifungals
- Cimetidine
- Selective serotonin reuptake inhibitors (SSRIs)
Carbamazepine is an enzyme-inducing drug which may accelerate the metabolism of some drugs resulting in their reduced therapeutic effect, such as:
- Corticosteroids
- Doxycycline
- Oral contraceptives
- Thyroid hormones
- Tricyclic antidepressants
- Simvastatin
- Warfarin
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
