Carbamazepine

Carbamazepine blocks the voltage-gated sodium channels of neurons. It keeps them in an inactivated state, which reduces the number of channels available at any given moment. Neurons then struggle to generate repeated action potentials, and the electrical runaway that triggers a seizure is checked at its source.

The liver transforms it, mainly through the enzyme CYP3A4, into an active metabolite: carbamazepine-10,11-epoxide. This molecule has one peculiarity: it induces its own metabolism. After a single dose, its plasma half-life is around 35 to 40 hours; with repeated dosing, this auto-induction brings it down to roughly 12 to 17 hours. The effective dose therefore changes during the first weeks, as the body adjusts.

In epilepsy, carbamazepine treats complex partial seizures, generalized tonic-clonic seizures, and mixed forms that combine these two types. Its action on sodium channels makes it effective where neuronal discharges spread and repeat, by calming the excitability of the cells involved.

It does not act on every type of seizure. On absence seizures, those brief lapses of consciousness, it remains ineffective and may even worsen them. The choice of molecule therefore depends on the type of epilepsy identified, which explains the weight given to a precise diagnosis before prescribing it.

Meal timing matters little: carbamazepine can be taken during, after, or between meals, since food does not significantly change the amount absorbed. The standard tablet and the suspension are usually spread over two to four doses per day, often at mealtimes; the extended-release form is taken twice a day, which smooths out the concentrations.

Consistency is key. The doctor adjusts the dose based on the blood concentration, whose therapeutic range usually lies between 4 and 12 micrograms per milliliter, the same in children and adults. This treatment is never stopped on one’s own: an abrupt halt risks a return of seizures. Any change goes through the doctor.

Some effects affect at least one person in ten, especially at the start of treatment: dizziness, drowsiness, ataxia (problems with coordination and balance), nausea, and vomiting. They often ease as the body gets used to it or when the dose is adjusted. On the biological side, leukopenia (a drop in white blood cells) and a rise in hepatic GGT, reflecting enzyme induction, are also very common; hyponatremia (a drop in blood sodium), often linked to SIADH, is common (between 1 in 100 and 1 in 10) and affects older people and patients on diuretics more.

Other effects, rarer, are serious and warrant close vigilance. Carbamazepine can cause severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), strongly associated with the HLA-B*1502 allele, hence the screening recommended in at-risk populations before starting. The risk of agranulocytosis and aplastic anemia is estimated to be 5 to 8 times higher than in the general population, where it remains low (on the order of 6 cases per million per year for agranulocytosis, 2 for aplastic anemia). Liver damage can occur, most often as part of a hypersensitivity syndrome (DRESS) appearing 1 to 8 weeks after the start.

Like other enzyme inducers, carbamazepine speeds up the breakdown of vitamin D in the liver. A drop in plasma calcium and in 25-hydroxycholecalciferol, the circulating form of vitamin D, is observed. In the long term, this impact on bone can lead to osteomalacia or osteoporosis, a very rare effect (fewer than 1 case in 10,000).

The treatment also affects folates. Folate deficiency is described as rare, and the megaloblastic anemia that can follow as very rare. In women, folic acid supplementation is recommended before and during pregnancy. These two aspects, bone and folates, outline the nutrients to watch under carbamazepine.

A potent enzyme inducer, carbamazepine speeds up the elimination of many medicines. It reduces the effectiveness of hormonal contraceptives, with reported breakthrough bleeding and unwanted pregnancies, and lessens the anticoagulant effect of warfarin and other oral anticoagulants. Conversely, CYP3A4 inhibitors such as erythromycin and grapefruit juice raise its plasma concentration, with a risk of overdose.

Laboratory monitoring frames these risks. Blood sodium is measured before treatment, then around two weeks, and thereafter every month during the first three months. A baseline and then periodic complete blood count monitors the blood cell lines, and regular checking of liver function completes the picture. This schedule allows the effects that matter to be spotted early.