Clinical Psychopharmacology · Paper A
Lithium side effects
Monitoring is built into every lithium prescription because the dose that treats maniaA period of abnormally elevated mood, energy and activity, seen in bipolar disorder.Read more → sits only a short distance below the dose that poisons the patient, and that narrow gap is why a level, not just a symptom, decides safety. The therapeutic band runs roughly 0.6 to 0.8 mmol/L, rising to 1.0 for acute mania: the over-elevated 'high' phase of bipolar disorder. Within that band patients commonly notice a FINE, fast hand tremor, pass more urine, feel thirsty, taste metal, gain weight, or feel nauseated when starting; all expected costs of being on a narrow-margin drug, not reasons to stop it. A patient who instead develops a coarse, large-amplitude tremor or new confusion has crossed into different territory, so the question a clinician always asks is which side of that line a symptom sits on.
Long-term lithium exposure affects several organs beyond the acute picture above. It can cause hypothyroidism, sometimes with a goitre: a swollen thyroid gland in the neck. It can cause nephrogenic diabetes insipidus, a kidney disorder where urine can no longer be concentrated because the collecting duct stops responding to ADH (antidiuretic hormone), plus a gradual fall in kidney filtering measured by the eGFR. It can raise blood calcium via overactive parathyroid glands, and cause a harmless flattened T-wave on the ECGElectrocardiogram, a recording of the heart's electrical activity.Read more →. Toxicity, a level above roughly 1.5 mmol/L, looks quite different: a COARSE, large-amplitude tremor, ataxiaLoss of full control of body movements, causing an unsteady, uncoordinated gait or actions.Read more → (unsteady, clumsy movements from loss of coordination), slurred speech, drowsiness progressing to confusion, seizures and kidney failure. For example, a stable patient who starts an NSAID for back pain can develop a coarse tremor with new unsteadiness days later, because the interacting drug pushed the level into the toxic range. Asking whether a tremor is fine or coarse, and checking for a recently added interacting drug, is what decides whether this is routine or an emergency.
Bipolar disorder needs a mood stabiliser, and lithium remains one of the most effective drugs psychiatry has for stopping the swings between and depression. But lithium is unusual among psychiatric drugs because the dose that treats the illness and the dose that poisons the patient are separated by only a small gap. This is what "narrow therapeutic index" means: the therapeutic window (the blood level range where the drug works) sits very close to the toxic window (the level where it starts to harm the body), so a small rise in blood level can tip a patient from "on treatment" to "in danger" within days. That is why every side effect on this list has to be filed under one of two boxes: expected and safe, or a warning sign, and getting that filing wrong is a genuine patient-safety failure, not just an exam trap.
A drug's compares the dose that helps to the dose that harms; a WIDE therapeutic index (like paracetamol at normal doses) gives a big safety cushion, while a NARROW one (like lithium, digoxin, warfarin) gives almost none. For lithium specifically, the therapeutic range (the blood concentration doctors aim to keep a patient within) is roughly 0.6-0.8 mmol/L, rising to 1.0 mmol/L when treating an acute manic episode (the over-elevated "high" phase of bipolar disorder). A side effect, in the plainest sense, is any unwanted bodily change caused by a drug at a dose meant to treat the illness: it is different from toxicity, which is harm caused specifically because the level has climbed too high. Confusing the two is exactly the caveat below.
The single most dangerous mistake with lithium is treating every tremor, every episode of nausea, or every bout of unsteadiness as "just a known side effect" and never checking a level. A FINE, fast tremor of the hands is a normal, expected finding at a therapeutic dose; a COARSE, large-swing tremor is a toxicity sign and needs an urgent blood level. The second common way this goes wrong is missing a drug interaction that quietly pushes the level up without any dose change at all: NSAIDs (non-steroidal anti-inflammatory drugs, like ibuprofen), ACE inhibitors (a class of blood pressure drugs), thiazide diuretics (a class of water tablets), and dehydration from any cause (vomiting, diarrhoea, a hot summer, reduced fluid intake in illness) all reduce how efficiently the kidney clears lithium, so the same daily dose starts to accumulate. A patient who "hasn't changed their lithium dose" can still be heading into toxicity if any of these were added, or if they became unwell and stopped drinking normally.
Lithium is not broken down by the liver like most psychiatric drugs: it is a simple ion, handled almost entirely by the kidney, and it is reabsorbed in the same part of the kidney tubule that reabsorbs sodium. That single fact explains almost everything on this card. When the body is short of sodium or fluid (dehydration, vomiting, diuretics), the kidney reabsorbs more sodium to compensate, and drags extra lithium back into the blood alongside it, because the kidney cannot tell the two apart. This is why sodium/fluid problems and certain drugs raise the level even though the patient took exactly the same tablets: the mechanism is "less drug removed", not "more drug in". Above roughly 1.5 mmol/L lithium becomes directly toxic to nerve and muscle tissue, producing the coarse tremor, (unsteady, uncoordinated movements from loss of muscle control) and confusion of toxicity, and at very high levels it can trigger seizures and kidney failure: a vicious cycle where kidney injury cuts clearance further and drives the level higher still.
Consider a patient stable on lithium with a checked level of 0.7 mmol/L: they mention a fine hand tremor, feel thirsty and pass more urine than usual, taste a metallic tang, and have gained a little weight since starting: all textbook EXPECTED effects at a therapeutic level, needing reassurance rather than a change in treatment. Beyond these early effects, chronic use can also cause hypothyroidism (an underactive thyroid gland), sometimes with a visible goitre (a swollen thyroid gland in the neck), gradually raised blood calcium from overactive parathyroid glands, and a slow decline in kidney function measured by the eGFR (estimated glomerular filtration rate, a blood test estimate of how well the kidneys filter). Now consider a second patient, on the same dose, who develops a cold and stops eating and drinking properly for two days: their tremor becomes coarse and their speech starts to slur, and a repeat level comes back at 2.2 mmol/L. That is toxicity: the same tablets, the same starting dose, but dehydration removed the safety margin. Recognising which patient is which, from the story alone, is the entire clinical skill this concept is testing.
(a dangerous drug reaction, usually from combining two drugs such as SSRIs) also causes tremor and confusion, but adds fever, sweating, diarrhoea and (overactive tendon reflexes) with a fast onset over hours after a drug change: lithium toxicity instead tracks a rising blood level over days and lacks that marked autonomic overactivity. malignant syndrome (a rare, dangerous reaction to antipsychotics) causes severe muscle rigidity and a very high fever, which coarse-tremor lithium toxicity does not produce. Simple dehydration alone, without a raised lithium level, can cause thirst and mild confusion in an elderly patient, but will not produce a coarse tremor or : the lithium level itself is what separates a benign fluid problem from a genuine emergency, which is exactly why the level, not the symptom pattern alone, is checked whenever any of these differentials is being considered.
Diagnosing lithium toxicity, and safely tracking therapeutic use, both rest on a fixed set of real investigations, not clinical impression alone. The lithium level itself is measured 12 hours after the last dose (a standardised trough sample so results are comparable between labs and patients), checked weekly after starting or after any dose change until stable, then every 3-6 months once established. Renal function is checked with (urea and electrolytes) and eGFR/creatinine before starting and at least every 6 months, since the kidney is both the organ that clears lithium and an organ lithium can damage. Thyroid function is checked with TSH (thyroid-stimulating hormone) before starting and every 6-12 months, because hypothyroidism can develop silently. Calcium is also checked periodically, since chronic lithium use can cause hyperparathyroidism. A patient reporting new symptoms should have an unscheduled level taken immediately rather than waiting for the routine 3-6 month check.
For expected, therapeutic-level side effects (fine tremor, mild polyuria, metallic taste), management is reassurance, continuing lithium, and confirming the level is in range: no drug change is usually needed. For genuine toxicity, first-line management is to stop lithium immediately, give supportive care (intravenous fluids to restore volume and improve renal clearance, plus correcting any electrolyte problem), and monitor closely; in severe toxicity (very high levels or severe neurological signs such as seizures or coma), haemodialysis removes lithium directly from the blood. This is not an invented protocol: the Maudsley Prescribing Guidelines in Psychiatry set out this exact monitoring schedule and toxicity ladder, and NICE guidance (NICE NG222, bipolar disorder in adults) requires patients starting lithium to carry a lithium alert card and be enrolled in a monitoring system, such as a lithium record book, because informal, symptom-only monitoring is not considered safe practice.
If a vignette gives you a tremor plus ANY of "started ibuprofen", "started a new blood pressure or water tablet", or "unwell and not drinking", think toxicity from reduced before you think "just a side effect": the exam is testing whether you spot the mechanism (less lithium being removed), not just the symptom.
Lithium's narrow gap between the dose that helps and the dose that harms means every side effect must be sorted into "expected" or "toxic", and that sorting is done with a blood level, not a guess.