Clinical Psychopharmacology · Paper A
Drug-specific side effects
This concept tests signature drug-to-side-effect links, the associations an examiner expects you to recognise instantly. The trick is to map a drug's receptor actions to its cluster of side effects. A stem describing dry mouth, blurred vision and constipation together should point straight to a tricyclic, before any drug name is given, because that anticholinergicBlocking the action of acetylcholine, a nerve chemical, causing effects like a dry mouth and blurred vision.Read more → cluster is its signature. Learning clusters, not isolated symptoms, is what actually speeds up exam recognition.
SSRISelective serotonin reuptake inhibitor, a common type of antidepressant.Read more → (selective serotonin reuptake inhibitor) antidepressants cause hyponatraemia (a low blood sodium, especially in the elderly), GIGastrointestinal, relating to the stomach and intestines.Read more → (gastrointestinal, stomach and gut) upset, sexual dysfunction and a slightly raised bleeding risk. Lithium causes a kidney problem with excess dilute urine, an underactive thyroid, a fine tremor and weight gain. Tricyclics cause an anticholinergicBlocking the action of acetylcholine, a nerve chemical, causing effects like a dry mouth and blurred vision.Read more → cluster (dry mouth, blurred near vision, constipation and difficulty passing urine) from blocking muscarinic receptorsA receptor for the nerve chemical acetylcholine, whose blockade causes anticholinergic side effects like dry mouth.Read more →, plus drowsiness, dizziness on standing, and dangerous heart rhythms in overdose. An elderly patient started on an SSRI who becomes confused two weeks later is a classic pattern: confusion with a low sodium should trigger a hyponatraemia check before any other cause is chased. Take a patient on long-term lithium who develops excess thirst and passes large volumes of dilute urine, e.g. this points to nephrogenic diabetes insipidus from lithium, not simple poor fluid intake. Clinically, spotting which receptor or organ a drug hits lets a clinician predict its side-effect cluster before waiting for blood results to confirm it.
A psychiatric exam stem rarely names the drug up front. Instead it describes a cluster of symptoms and expects the candidate to work backwards to the drug, or forwards from a named drug to its most dangerous side effect. This is a pattern-recognition skill, not a memory dump of every possible adverse event: examiners test the SIGNATURE (the one or two side effects that are distinctive enough to identify a drug class on their own), not the long tail of rare reactions.
An ADVERSE DRUG REACTION (an unwanted or harmful effect caused by a medicine at a normal dose) can be predictable, dose-related, and mechanistic, or idiosyncratic and unrelated to dose. Most of the reactions tested at this level are mechanistic: they follow directly from which RECEPTOR (a protein that a drug binds to, triggering or blocking a biological effect) the drug acts on. MONITORING means a planned schedule of blood tests, physical checks or symptom review used to catch a side effect early, before it becomes dangerous.
A common trap is treating side effects as an unordered list to memorise. Suppose a stem describes a patient on lithium with a fine hand tremor, excessive thirst and a large volume of dilute urine: a candidate who has only memorised "lithium causes tremor" in isolation may miss that the thirst and polyuria point to nephrogenic diabetes insipidus, a specific and testable renal effect, not a vague catch-all "kidney problem".
Start from receptor pharmacology. A drug that blocks produces the cluster: dry mouth, blurred near vision, constipation and urinary retention. A drug that blocks histamine receptors sedates and drives weight gain. A drug that blocks receptors causes a drop in blood pressure on standing. Once the receptor-to-symptom map is fixed in mind, an unfamiliar drug's side-effect profile can often be predicted rather than recalled.
SSRIs (selective serotonin reuptake inhibitors) cause gastrointestinal upset, sexual dysfunction, a small increase in bleeding risk, and hyponatraemia (a low blood sodium), which is especially important in older adults. Lithium causes a fine tremor, weight gain, an underactive thyroid, and nephrogenic diabetes insipidus. Tricyclic antidepressants cause the cluster plus sedation, postural hypotension, and, in overdose, life-threatening cardiac arrhythmias from sodium-channel blockade. Clozapine carries the most feared specific risk: agranulocytosis (a dangerous fall in neutrophil white blood cells), alongside myocarditis, severe constipation that can progress to bowel obstruction, and a lowered seizure threshold. Sodium valproate causes weight gain, tremor, hair loss and is strongly in pregnancy. Carbamazepine causes hyponatraemia, skin rashes, and induces liver enzymes that speed up the breakdown of many other drugs, including the combined oral contraceptive pill.
Do not confuse lithium's tremor with the coarse, high-amplitude tremor of lithium toxicity, which comes with vomiting, (unsteady gait) and confusion and is a medical emergency, not a routine side effect. Do not confuse clozapine's usual mild neutrophil dip with true agranulocytosis, which needs an urgent full blood count to distinguish. Do not confuse -induced hyponatraemia with dehydration; the confirmatory investigation is a serum sodium plus urine osmolality, not clinical impression alone.
Diagnosis of a drug-specific reaction rests on temporal association with starting or increasing the drug, plus the relevant investigation: a full blood count for clozapine-related agranulocytosis, serum lithium level and renal or thyroid function tests for lithium, and serum sodium for or carbamazepine-related hyponatraemia. Baseline and ongoing screening (weight, lipids and glucose for antipsychotics, thyroid and renal function for lithium) is part of routine management, not an optional extra.
Management is drug-specific. Suspected agranulocytosis on clozapine means stopping clozapine immediately and arranging an urgent full blood count, following Maudsley Prescribing Guidelines and clozapine-service protocols. Lithium toxicity needs the level checked, the dose withheld, and the cause of dehydration or interacting drug identified. side effects can sometimes be managed by lowering the dose or switching to a less anticholinergic drug in the same class. NICE guidance on medicines optimisation underpins the general principle of reviewing risk and benefit at every dose change.
Fever plus sore throat on clozapine is agranulocytosis until an urgent full blood count proves otherwise. Tremor plus polyuria and polydipsia on lithium points to nephrogenic diabetes insipidus. Confusion with a low sodium in an older adult on an should trigger a hyponatraemia check before any other cause is chased. A rash on carbamazepine or lamotrigine, especially with fever, should raise concern for a severe cutaneous reaction and prompt the drug's withdrawal.
Every drug's side-effect profile follows logically from the receptors it acts on, and each major psychiatric drug has one or two signature reactions (agranulocytosis for clozapine, nephrogenic diabetes insipidus for lithium, hyponatraemia for SSRIs and carbamazepine, teratogenicity for valproate) that an exam stem will lean on to identify the drug without naming it, so learning the receptor-to-cluster map pays off far more than memorising an unordered list of possible effects.