Lithium and carbamazepine: the neurotoxicity warning explained

Lithium is described by the NHS as a mood stabiliser. It is used for mania, hypo-mania, regular periods of depression where other treatments have not worked, and bipolar disorder. It can also help to reduce aggressive or self-harming behaviour.

Carbamazepine is best known as an epilepsy medicine, and the NHS carbamazepine page lists use for epilepsy, facial pain from trigeminal neuralgia and, if other medicines have not worked, bipolar disorder. In specialist practice both can be prescribed, which means a combination is sometimes intentional and sometimes arises because separate clinicians treat separate conditions.

Either way, the combination is one where the monitoring plan matters more than any single rule, because both medicines can affect the nervous system.

In the BNF interaction list for lithium, the carbamazepine entry states that carbamazepine is predicted to increase the risk of neurotoxicity when given with lithium. The manufacturer makes no recommendation, the severity is severe and the evidence type is anecdotal.

That mix of wording is worth reading carefully. Severe means the potential effect is serious. Anecdotal means the evidence comes mainly from case reports rather than large trials. And no manufacturer recommendation means there is no simple instruction such as avoid or adjust dose, so the judgement sits with the specialist who knows the person.

The NHS says that, although it is rare, too much lithium in the blood can cause problems with the kidneys and other organs. It lists symptoms that should prompt a call to 999 or a trip to A&E: stomach ache, nausea and diarrhoea; blurred vision; feeling very thirsty, needing to pee more than normal and lack of control over pee or poo; feeling lightheaded or drowsy; confusion and blackouts; shaking, muscle weakness, twitches, jerks or spasms affecting the face, tongue, eyes or neck; and difficulty speaking.

The NHS says these are signs of lithium toxicity, which is an emergency, and says to stop taking lithium straight away and not to drive to A&E. This site has a separate overview of lithium toxicity for readers who want more detail.

• Stomach ache, feeling sick or diarrhoea.

• Blurred vision.

• Being very thirsty and passing much more urine than usual.

• Drowsiness, confusion or blackouts.

• Shaking, muscle weakness or twitching, and difficulty speaking.

The NHS says to go to the blood tests arranged by your doctor, not to reduce your salt intake suddenly, to drink plenty of fluids, particularly in hot weather or during intense exercise, and to tell any doctor or pharmacist that you take lithium before starting any new medicine. It says a low-sodium diet can increase lithium levels.

Keep the lithium record book and alert card up to date. If carbamazepine is started, stopped or its dose changed, ask whether an extra lithium level check is needed. The sources read do not specify timing, so that is for the prescriber to decide.

In the NHS, lithium is generally managed by mental health services with GP involvement, and community pharmacists can check any new prescription against the BNF. Private psychiatrists use the same reference, so the full list of medicines should be shared with them too. Never stop either medicine abruptly. The NHS says that if you take carbamazepine for seizures, you should not stop it without talking to a doctor first, because seizures may return.