Clarithromycin × Metformin

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.

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.

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.