NEW Added to close a syllabus gap — the syllabus says "drugs used, especially in anaesthesia and in internal medicine". Easy to skip.

Drugs from internal medicine

The chronic medications your patients arrive on — what they do, whether to continue them, and how they interact with anaesthesia.

Cardiovascular

Class	Mechanism	Perioperative
Beta blockers	β₁ blockade → ↓rate, ↓contractility, ↓renin	CONTINUE — abrupt withdrawal causes rebound ischaemia and arrhythmia. Do not start acutely on the day (POISE: more strokes and deaths)
ACE inhibitors / ARBs	Block angiotensin II; ACEi also raises bradykinin (cough, angioedema)	Consider omitting on the morning of surgery — refractory hypotension. Treat with vasopressin if unresponsive to catecholamines
Calcium channel blockers	Dihydropyridines vasodilate; verapamil and diltiazem are rate-limiting	Continue. Potentiate neuromuscular blockade
Statins	HMG-CoA reductase inhibitors; pleiotropic plaque stabilisation	CONTINUE — withdrawal is associated with worse cardiac outcome. Watch for rhabdomyolysis
Diuretics	See the renal note for sites of action	Usually omit on the day; check K⁺ and volume status

Drug	Issue	Action
Sulphonylureas	Stimulate insulin release independently of glucose → hypoglycaemia when fasted	Omit on the day of surgery
Metformin	Does not cause hypoglycaemia; risk is lactic acidosis with renal impairment or contrast	Usually continue unless renal impairment or contrast planned
SGLT2 inhibitors	Euglycaemic DKA — a normal glucose does not exclude it	Withhold ~3 days preoperatively
Long-term steroids	HPA axis suppression → addisonian crisis	Continue and give supplementary hydrocortisone scaled to surgical magnitude
Levothyroxine / carbimazole	Thyroid status determines risk	Continue. Render euthyroid before elective surgery

Antiepileptics — continue without interruption. Phenytoin shows zero-order kinetics and is a potent enzyme inducer; carbamazepine and phenytoin both shorten the duration of non-depolarising blockers through induction and receptor upregulation.
MAOIs — the classic danger. Never give pethidine (serotonin syndrome: hyperthermia, rigidity, agitation, cardiovascular collapse). Avoid indirect sympathomimetics such as ephedrine; use small doses of a direct agent like phenylephrine.
SSRIs — serotonin syndrome risk with pethidine, tramadol and methylene blue. They also impair platelet function and increase bleeding.
Tricyclics — anticholinergic effects, arrhythmias, and exaggerated response to sympathomimetics.
Lithium — narrow therapeutic index; prolongs neuromuscular blockade; toxicity precipitated by dehydration, NSAIDs, diuretics and ACE inhibitors. Check the level.
Parkinson's medication — continue; abrupt withdrawal risks a neuroleptic-malignant-like syndrome. Avoid metoclopramide, droperidol and haloperidol (dopamine antagonists).
Myasthenia treatment — see the dedicated note; anticholinesterases interact with relaxants and with ester local anaesthetics.

Inhaled β₂ agonists and steroids — continue, and give a preoperative dose. Theophylline has a narrow therapeutic index and interacts with adenosine (antagonism) and with enzyme inhibitors.
Proton pump inhibitors and H₂ blockers — useful as antacid prophylaxis; omeprazole is an enzyme inhibitor.
Immunosuppressants (ciclosporin, tacrolimus, azathioprine, biologics) — continue transplant immunosuppression. Ciclosporin is nephrotoxic and potentiates relaxants; azathioprine antagonises non-depolarising blockade.
Herbal medicines — the "4 Gs": Garlic, Ginkgo, Ginseng, Ginger increase bleeding. St John's wort is a potent enzyme inducer. Stop 2 weeks preoperatively.

Full detail is in the Coagulation & Transfusion note — mechanisms, monitoring and reversal.
The perioperative question is always the same trade-off: thrombotic risk of stopping versus bleeding risk of continuing, and for neuraxial blockade the timing intervals from the current national guideline.
Aspirin is usually continued for secondary prevention. Clopidogrel is typically stopped 7 days before, unless a recent coronary stent makes that unsafe — discuss with cardiology.