Why dangerous: Metformin is renally cleared; accumulation blocks hepatic lactate use and causes lactic acidosis.

Instead / what to do:

• eGFR < 30 mL/min/1.73 m²: contraindicated, stop.
• eGFR 30–45: do not start; if already on it, review and reduce dose (max about 1 g/day).
• Withhold during sepsis, shock, dehydration, hypoxia or acute kidney injury (sick-day rule).
• Alternatives in CKD: DPP-4 inhibitor (linagliptin needs no dose change), GLP-1 RA, SGLT2 inhibitor (for kidney/heart benefit), insulin.

Why dangerous: Contrast-associated AKI can make metformin accumulate and trigger lactic acidosis.

Instead / what to do:

• Check eGFR before contrast in diabetics/CKD.
• Hold metformin at the time of contrast if eGFR < 30, AKI, or intra-arterial contrast; restart after 48 h once creatinine is stable.
• eGFR ≥ 30 and no AKI: most guidelines (ACR) allow continuing; exams often still expect "hold 48 h".
• Prevent contrast AKI: IV isotonic saline, lowest contrast dose, stop NSAIDs/nephrotoxins.

Why dangerous: SGLT2 inhibitors lower glucose through urinary loss, so ketoacidosis can develop with glucose below 250 mg/dL (14 mmol/L).

Instead / what to do:

• Stop canagliflozin, dapagliflozin, empagliflozin 3 days before planned surgery (ertugliflozin 4 days); stop during acute illness, vomiting or ketogenic diet.
• Euglycaemic DKA: IV fluids + insulin with dextrose from the start, potassium replacement, stop the drug.
• Other SGLT2i traps: genital mycotic infection, volume depletion, Fournier gangrene (rare).