Nursing care
Metformin: what to check before you give it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Metformin is held for 48 hours before and after iodinated contrast studies because of the risk of contrast-induced kidney injury leading to lactic acidosis, not because of hypoglycaemia. On its own, metformin rarely causes low blood glucose, since it works by reducing hepatic glucose output rather than stimulating insulin release.
What it does and why it is prescribed
Metformin is the first-line oral agent for type 2 diabetes. It lowers blood glucose mainly by reducing glucose production in the liver and improving peripheral insulin sensitivity, rather than by pushing the pancreas to release more insulin. It is also prescribed off-label for polycystic ovary syndrome and, in some practices, for prediabetes.
Because it does not stimulate insulin secretion, metformin carries a very low risk of causing hypoglycaemia when used alone. That single mechanism explains most of the drug's safety profile and most of the nursing decisions built around it, including the contrast rule covered below.
Nursing considerations before giving it
Check renal function before every new prescription and periodically thereafter, since metformin is renally cleared and accumulates if the kidneys cannot excrete it. Reduced clearance is the mechanism behind the drug's rare but serious complication, so an eGFR or creatinine result out of range is a reason to question the order rather than administer as usual.
Ask specifically about any imaging study involving iodinated contrast planned in the next 48 hours, or any that occurred in the last 48 hours. This is the single most important scheduling question for this drug, and it is easy to miss on a busy ward round because contrast studies are ordered by radiology, not by the prescriber managing diabetes. Give with food to reduce the gastrointestinal upset that is otherwise the most common reason patients stop taking it.
What to monitor
Monitor renal function regularly, since deteriorating kidney function is what turns a safe drug into a dangerous one. Monitor blood glucose and HbA1c to assess whether the dose is achieving glycaemic control, but do not expect the dramatic swings seen with insulin or sulfonylureas.
Watch for early signs of lactic acidosis: unexplained muscle pain, marked fatigue, difficulty breathing, abdominal discomfort, or a sudden drop in body temperature. These can be subtle and are easy to attribute to something else in a patient who is already unwell, which is exactly why vigilance matters in anyone with reduced renal function, sepsis, dehydration, or recent contrast exposure. Vitamin B12 levels are worth checking periodically with long-term use, since metformin can reduce absorption.
Side effects versus adverse effects
Common side effects are gastrointestinal: nausea, diarrhoea, abdominal discomfort, and a metallic taste, most pronounced when starting treatment or increasing the dose. These usually settle with time, taking the dose with food, or using an extended-release formulation, and rarely require stopping the drug outright.
The adverse effect that defines this drug's nursing management is lactic acidosis, a rare but potentially fatal complication that occurs when metformin accumulates in the setting of impaired renal clearance, tissue hypoxia, or severe dehydration. This is precisely why the drug is held around contrast studies: the contrast itself can transiently reduce renal function, and if metformin is still on board while clearance drops, lactic acid can accumulate. Unlike sulfonylureas or insulin, metformin does not itself cause hypoglycaemia, so a low glucose reading in a patient on metformin alone should prompt a look for another cause.
What to hold for and when to call
Hold metformin for 48 hours before and 48 hours after any procedure using iodinated contrast, and confirm renal function has returned to baseline before restarting it. This is a fixed rule, not a judgement call, and it applies regardless of how well controlled the patient's diabetes is otherwise.
Hold the dose and call the prescriber for any acute illness involving vomiting, diarrhoea, or reduced oral intake, since dehydration raises the risk of lactic acidosis, and for any new evidence of declining renal function. Call promptly, and treat as an emergency, if a patient on metformin develops unexplained muscle pain, hyperventilation, drowsiness, or hypotension, since these can be early signs of lactic acidosis and need urgent blood gas and lactate testing.
Patient teaching
Teach the patient to take metformin with food to limit stomach upset, and to expect that gastrointestinal symptoms are usually worst in the first few weeks. Explain that unlike some other diabetes medicines, metformin on its own is very unlikely to cause a hypoglycaemic episode, so they do not need to carry glucose tablets purely because of this drug, though they should still know the signs if they are on other agents too.
The teaching point they must retain is the contrast rule: tell every radiology department, every new doctor, and every dentist doing sedation imaging that they take metformin, and expect it to be stopped around any scan involving contrast dye. Tell them to report persistent vomiting, diarrhoea, or an inability to keep fluids down promptly, since dehydration is the other main trigger for the drug's rare serious complication, and to attend all scheduled blood tests for kidney function.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our endocrine practice questions are the closest set to what this page covers.
Common questions
Why is metformin held before a CT scan with contrast?
Iodinated contrast can transiently impair renal function, and metformin is cleared by the kidneys. If it accumulates because clearance drops, it raises the risk of lactic acidosis, so it is held for 48 hours before and after and only restarted once renal function is confirmed normal.
Does metformin cause hypoglycaemia?
Rarely, when used alone. Metformin lowers blood glucose by reducing hepatic glucose production rather than stimulating insulin release, so a low reading in a patient on metformin monotherapy should prompt a search for another cause.
What are the early signs of metformin-associated lactic acidosis?
Unexplained muscle pain, marked fatigue, rapid or laboured breathing, abdominal pain, and low body temperature. These signs can be subtle, so any of them in a patient with reduced renal function or recent contrast exposure warrants urgent assessment and lactate testing.
Can metformin be given on an empty stomach?
It is better given with food, since this reduces the gastrointestinal upset, nausea and diarrhoea, that is the most common reason patients discontinue the drug. It does not need to be timed to a meal the way rapid-acting insulin does.
What should be checked before starting metformin?
Baseline renal function, since the drug is renally cleared and dose or suitability depends on it. Any planned contrast imaging should also be identified in advance so the dose can be scheduled around it.