Metformin

Mechanism

Intravenous iodinated contrast media (iopromide) can cause acute kidney injury via osmotic load and direct toxicity. On metformin (renally excreted), plasma levels rise and lactic acidosis risk accumulates – a rare but life-threatening complication.

Symptoms

Lactic acidosis: dyspnoea, muscle pain, nausea, confusion, hypotension. Laboratory: blood pH below 7.35 and lactate above 5 mmol/L. Symptoms appear 1–3 days after contrast.

Management

For planned contrast studies, withhold metformin on the day of the procedure. Resume 48 hours later only with stable renal function (creatinine check before resuming). Alternative imaging in renal impairment: non-gadolinium MRI.

Mechanism

Prednisolone (a systemic glucocorticoid) suppresses muscle glucose uptake and stimulates hepatic gluconeogenesis – steroid-induced diabetes develops or existing type 2 diabetes decompensates. Metformin offsets part of the effect but not all.

Symptoms

Hyperglycaemia: thirst, polyuria, fatigue, rising fasting glucose and HbA1c. With high prednisolone doses: decompensation risk with ketoacidosis in type 1 diabetes.

Management

Monitor glucose 4 times daily during the first week of combination, then by self-monitoring. Increase metformin to maximum (2000–2500 mg/day) as needed. If decompensation occurs, add an α-glucosidase inhibitor, a GLP-1 receptor agonist, or temporary insulin.

Mechanism

Fluoroquinolones disturb glucose homeostasis — they can cause both hypoglycaemia (especially in older patients on glucose-lowering therapy) and hyperglycaemia. The mechanism involves pancreatic beta-cell effects.

Symptoms

Hypoglycaemia: sweating, tremor, hunger, confusion at glucose below 3.9 mmol/L. Hyperglycaemia: thirst, polyuria, fatigue.

Management

For short ciprofloxacin courses (5–7 days) on metformin, intensify glucose self-monitoring (twice daily) for the first week. In older patients and diabetics on insulin or sulfonylureas, choose an alternative antibiotic (cephalosporin, amoxicillin-clavulanate).

Mechanism

The metformin label does not name clarithromycin, and the risk lies elsewhere: metformin is cleared unchanged by the kidney, and its label warns of lactic acidosis when renal function drops. An infection with fever, vomiting or diarrhoea causes dehydration and lowers filtration, so the danger comes from the illness being treated rather than from the antibiotic itself.

Symptoms

Nausea, vomiting, abdominal pain, deep rapid breathing, muscle cramps, drowsiness: this is how lactic acidosis starts, and it is mistaken for the infection itself. Clarithromycin brings a metallic taste and digestive upset, which is a different thing.

Management

Metformin continues while the patient eats and drinks normally. If vomiting, diarrhoea or high fever prevent hydration, hold it temporarily and restart once intake and renal function are back to normal, as the label directs for iodinated contrast procedures.

Mechanism

Metformin suppresses calcium-dependent absorption of the B12-intrinsic factor complex in the terminal ileum. With over 4 years of therapy, clinically significant B12 deficiency is reported in 10–30% of patients.

Symptoms

Fatigue, pallor, paraesthesias in hands and feet, gait instability, memory decline. Develops over 2–4 years of unrecognised therapy.

Management

On long-term metformin, check B12 yearly. If it falls below 200 pg/mL, give cyanocobalamin 500–1000 mcg/day orally or 1000 mcg/week intramuscularly (in significant neuropathy). Supplementation does not cancel the metformin indication.

Mechanism

Dexamethasone stimulates gluconeogenesis and reduces tissue insulin sensitivity. The glycaemic effect is stronger than with prednisolone due to long half-life and high potency.

Symptoms

Return of thirst and polyuria, rising fasting glucose, fatigue. In type 2 diabetes, possible decompensation with HbA1c rise of 0.5–1.5%.

Management

For short dexamethasone pulses (1–3 days, e.g. cerebral oedema or post-chemotherapy), no metformin adjustment needed — transient hyperglycaemia is tolerated. For long-term therapy, increase metformin or add long-acting insulin. Check glucose twice daily in the first week.

Mechanism

ACE inhibitors improve tissue insulin sensitivity and potentiate metformin's glucose-lowering effect. They do not cause hypoglycaemia alone but lower the threshold on insulin or sulfonylureas.

Symptoms

Signs of hypoglycaemia: sweating, tremor, hunger, tachycardia, confusion. Symptoms appear at glucose below 3.9 mmol/L.

Management

On metformin alone, no dose adjustment needed. In patients on insulin or sulfonylureas, intensify glucose self-monitoring for the first 2 weeks after starting enalapril. The effect is helpful in type 2 diabetes with arterial hypertension.

Mechanism

Thiazide diuretics reduce glycaemic control via hypokalaemia-driven suppression of insulin secretion. HbA1c may rise by 0.3–0.5%, especially in the first 6 months.

Symptoms

Return of thirst and polyuria, rising fasting glucose, signs of worsening control. Symptoms are typically mild.

Management

Check fasting glucose and HbA1c 3 months after starting hydrochlorothiazide. If HbA1c rises, increase metformin or add another glucose-lowering agent. Potassium every 6 months. In overt diabetes decompensation, alternatives: indapamide (less metabolic effect) or an SGLT-2 inhibitor.

Mechanism

There is no direct pharmacokinetic interaction: in the FDA label, ibuprofen 400 mg did not change the concentration of metformin 850 mg, and metformin did not change ibuprofen. The risk runs through the kidney. Ibuprofen, like other non-steroidal anti-inflammatory drugs (NSAIDs), inhibits the synthesis of prostaglandins that maintain renal blood flow. In vulnerable patients it causes fluid retention and an acute drop in kidney function. The kidney clears metformin, so when the glomerular filtration rate (GFR) falls the drug accumulates. Metformin accumulation is the main risk factor for lactic acidosis. The metformin label names NSAIDs directly among drugs that can acutely impair kidney function and raise the risk of lactic acidosis.

Symptoms

The first signal is worsening kidney function: rising creatinine and falling glomerular filtration rate (GFR), less urine, oedema from fluid retention. Elderly patients and those with dehydration, heart failure or diuretic treatment are most at risk. Lactic acidosis starts quietly: acidotic dyspnoea, abdominal pain, muscle cramps, weakness and hypothermia, then reduced consciousness up to coma. Laboratory findings: blood pH below 7.35, lactate above 5 mmol/L, a widened anion gap.

Management

The combination is acceptable under kidney monitoring. Ibuprofen is started with caution in a patient on metformin (metformin label, 4.4), and kidney function is followed closely while both are used (metformin label, 4.5). Glomerular filtration rate (GFR) is measured before metformin is started and then regularly: at least once a year, and every 3–6 months in the elderly and in patients at risk of progressive kidney disease (metformin label, 4.2 and 4.4). At GFR below 30 mL/min both drugs are contraindicated (metformin and ibuprofen labels, 4.3). Ibuprofen is given at the lowest dose for the shortest time, with monitoring, especially in heart, liver or kidney disease, on diuretics and in the elderly (ibuprofen label, 4.4; FDA ibuprofen label, Renal Effects). With dehydration from diarrhoea, vomiting, fever or low fluid intake, metformin is paused (metformin label, 4.4). Ibuprofen is contraindicated in severe dehydration (ibuprofen label, 4.3). No pharmacokinetic dose change is needed: ibuprofen 400 mg does not change metformin 850 mg exposure and vice versa (FDA metformin label, 12.3). If lactic acidosis is suspected (acidotic dyspnoea, abdominal pain, muscle cramps, weakness, hypothermia), metformin is stopped and the patient needs immediate medical care (metformin label, 4.4).

Mechanism

ARBs, like ACE inhibitors, improve tissue insulin sensitivity. In patients on insulin or sulfonylureas with metformin, the glucose-lowering effect may be amplified.

Symptoms

Signs of hypoglycaemia: sweating, tremor, hunger, tachycardia, confusion. Symptoms appear at glucose below 3.9 mmol/L.

Management

On metformin alone, no dose adjustment needed. On insulin or sulfonylureas, intensify glucose self-monitoring for the first 2 weeks after starting losartan. The effect is helpful in type 2 diabetes with arterial hypertension.

Mechanism

The metformin label states it outright: long-term use reduces absorption and serum levels of vitamin B12, and it advises considering this cause in megaloblastic anaemia. The mechanism is calcium-dependent in the terminal ileum, and the effect grows with dose and years of treatment.

Symptoms

Paraesthesia in the feet, unsteady gait, loss of vibration sense. In diabetes the picture is mistaken for diabetic polyneuropathy and treated as such for years while the deficiency continues.

Management

Measure B12 at least yearly in anyone more than four years on metformin, and whenever neuropathy or anaemia appears. A low value does not force metformin withdrawal: supplement B12 orally and keep the antidiabetic. Test as well in patients who also take a proton pump inhibitor, since the two mechanisms add up.

Mechanism

No direct pharmacokinetic interaction. Vitamin C increases iron absorption, not metformin's.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No dose adjustment needed. In diabetics on metformin, ascorbic acid may improve plant-source iron absorption in patients with deficiency.

Mechanism

Calcium carbonate slightly reduces metformin intestinal absorption when taken together. Clinically significant hyperglycaemia has not been described.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

Take metformin with or just after meals; take calcium carbonate 2 hours after meals or in the evening. On the combination, monitor HbA1c on the usual schedule. In diabetics with osteoporosis, the combination is standard.

Mechanism

Metformin on long-term use (over 5 years) moderately reduces blood B12 (by 10–30%). Effect on folate is minimal. Folate deficiency on metformin has not been described.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No folate dose adjustment needed. On long-term metformin, check B12 yearly. For pregnancy planning in women with diabetes, folic acid 400–800 mcg/day is standard regardless of metformin.

Mechanism

Omeprazole weakly inhibits intestinal OCT1, the transporter mediating metformin absorption. Metformin bioavailability may rise slightly. Clinical significance is minimal.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No dose adjustment needed. On long-term PPI therapy (over 2 years), check B12 yearly — both drugs may reduce its absorption.

Mechanism

Simethicone is a surface-active silicone agent that is not absorbed in the gastrointestinal tract. No direct pharmacokinetic interaction with metformin.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No dose adjustment needed. In diabetics with bloating (a metformin side effect), simethicone helps symptomatically. The combination is standard.

Mechanism

Zinc theoretically competes with metformin for renal tubular organic cation secretion, but no clinically significant hyperglycaemia has been described on the combination.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

No dose adjustment needed. In diabetics with zinc deficiency (e.g. on a vegan diet), zinc supplements 15–25 mg/day are safe. Monitor HbA1c on the usual schedule.