Metformin: what it is used for, dosage and side effects

What metformin is and which active substance it contains

Metformin contains metformin hydrochloride, the only member of the biguanide group still in general use. In the Maltese register of medicines it is recorded with metformin hydrochloride as its active substance and the oral route of administration. It is not insulin, it is not injected, and it works in a different way from every other group used in type 2 diabetes.

The register also carries a long list of fixed combinations that contain metformin hydrochloride together with a second substance, among them combinations with dapagliflozin, empagliflozin, linagliptin and sitagliptin. This matters in practice for one reason above all: if you already take a combination tablet, adding plain metformin on top of it means taking the same substance twice. Check the composition on the pack, not the name, and ask a pharmacist if you are not sure.

The substance is marketed by many companies under their own names and in more than one form, including modified-release tablets designed to be absorbed more gradually. Modified-release tablets are swallowed whole and are not halved or crushed unless the leaflet for your pack says otherwise, because breaking them destroys the mechanism that spreads the release out. Authorised products in Malta and their product information are listed by the Medicines Authority.

What it is used for

Metformin is used in type 2 diabetes, on its own or together with other medicines for diabetes, including insulin. It is generally the first medicine offered when diet and physical activity have not brought blood sugar to target, and it usually stays in the treatment when something else is added rather than being replaced. Which combination is right for you is decided by the doctor on the basis of your test results, your other conditions, your kidney function and your weight.

The medicine does not replace diet and activity, and this is not a polite formality: what you eat, how much you move and your body weight change how well any diabetes treatment works. Nor does it remove the need for the checks that go with the disease, because targets are set on measurements, not on how you feel. Type 2 diabetes very often produces no symptoms for years, which is exactly why treatment is judged by results rather than by sensation.

What the substance does depends on the condition it is used for, and the authorised indications for your product are listed in its product information rather than being something we can summarise for every pack. How the disease presents and which checks belong to it are described at /health-guide/type-2-diabetes-symptoms/, and the groups of medicines used alongside metformin at /diabetes-and-blood-sugar/.

How metformin works

Metformin acts mainly on the liver, reducing the amount of glucose the liver releases into the blood between meals and overnight. It also makes muscle and other tissues respond better to the insulin the body already produces, and it slightly reduces the absorption of glucose from the bowel. It does not push the pancreas to release more insulin, and that single fact explains most of what follows.

Because it does not force insulin out of the pancreas, metformin taken on its own does not usually cause hypoglycaemia. The risk of a low blood sugar appears when it is combined with insulin or with a sulfonylurea, and in those combinations the doctor often adjusts the dose of the other medicine rather than the metformin. Knowing the signs of a low blood sugar, such as trembling, sweating, confusion and sudden hunger, is part of treatment when those combinations are used.

The effect builds up rather than appearing with the first tablet, and it is measured in laboratory results. The doctor judges it from HbA1c, a blood test that reflects average blood sugar over a period of weeks, and compares it with the result before treatment. A single reading from a home meter says very little on its own; a series of readings and a laboratory result say a great deal.

The substance is removed from the body by the kidneys. That is why kidney function governs the dose, why it is rechecked from time to time, and why anything that suddenly reduces kidney function matters more with this medicine than with many others.

How it is taken and what to watch for

The dose is set by the doctor and written in the leaflet supplied with your pack. The usual approach is to begin low and increase in steps, because the digestive complaints that put people off metformin are far more likely when treatment starts at a higher dose. Tablets are taken with or just after a meal, which also reduces stomach upset, and modified-release forms are swallowed whole.

The medicine is not used where kidney function is severely reduced, in acute conditions that can affect the kidneys such as severe dehydration, severe infection or shock, in acute or unstable heart failure, in severe liver disease, in acute alcohol intoxication or alcohol dependence, or in a state of diabetic ketoacidosis or pre-coma. Those are contraindications, not reasons to be more careful.

There are situations in which the doctor tells you to pause the medicine for a while, and they are worth knowing in advance: before an examination using an iodine-containing contrast agent, before surgery under general or spinal anaesthesia, and during any illness that causes vomiting, diarrhoea or fever severe enough to leave you dehydrated. In each case the doctor also says when it may be restarted. Do not decide either the pause or the restart yourself.

The reason for this caution is lactic acidosis, a rare but serious build-up of acid in the blood that occurs almost always where kidney function has fallen or where one of those contraindications is present. Muscle cramps with stomach pain, deep or laboured breathing, marked weakness, feeling cold and severe drowsiness are its warning signs: stop the medicine and seek urgent medical help. In pregnancy, when planning a pregnancy and while breastfeeding, the treatment is reviewed by the doctor.

Interactions with other medicines

The interactions that matter most are those that reduce kidney function or cause dehydration, because both raise the level of metformin in the blood. Diuretics, particularly loop diuretics, non-steroidal anti-inflammatory medicines, and blood pressure medicines from the angiotensin-converting enzyme inhibitor and sartan groups belong here, especially when several are taken together during an illness with vomiting or diarrhoea.

Iodine-containing contrast agents used in imaging are the best known example of a temporary pause, and the doctor who orders the examination decides when the medicine stops and when it restarts. Corticosteroid tablets, beta-2 agonists in higher doses and some diuretics raise blood sugar, so blood sugar may need watching more closely while they are used.

With insulin and with sulfonylureas the risk of hypoglycaemia rises, and the doctor usually adjusts those medicines rather than the metformin. Alcohol deserves separate mention, because it both increases the risk of lactic acidosis and makes blood sugar less predictable, particularly on an empty stomach.

List for your doctor and your pharmacist everything you take, including medicines bought without a prescription, food supplements and herbal products. This is a medicine that is very often taken alongside several others, so a single review of the whole list by a pharmacist is more useful than checking one item at a time.

Side effects and when to seek help

The complaints described most often are digestive: nausea, diarrhoea, abdominal pain, loss of appetite and a metallic taste in the mouth. They are usually strongest at the start of treatment or after an increase in dose, and they ease in many people. Taking the tablet with food, increasing the dose slowly and, where the doctor agrees, a modified-release form all make them less likely.

With long-term use, absorption of vitamin B12 can fall, which is why the doctor may check it from time to time, particularly if you become unusually tired or develop tingling in the hands or feet. Skin reactions such as rash and itching are described less often.

Lactic acidosis is rare and serious. Muscle cramps together with stomach pain, deep or difficult breathing, marked weakness, feeling cold, an unusually slow heartbeat and severe drowsiness are its warning signs: stop the medicine and seek urgent medical help, and if the situation is severe call 112 or go to Accident and Emergency. The same applies to signs of a severe allergic reaction, such as swelling of the face, lips or tongue with difficulty breathing.

The complete list of side effects with information on how often each occurs is in the leaflet supplied with your pack. A suspected side effect can be reported to your doctor or pharmacist, and safety of medicines on the Maltese market is monitored by the Medicines Authority.

Prescription status and what we can do

Metformin is a prescription-only medicine in Malta, so the decision to start it and the dose belong to a doctor. We do not sell medicines and we cannot send you one. What we offer is a written medical questionnaire and a doctor review of your answers. There is no video call, no telephone consultation and no live chat.

We are not part of any Maltese state health record and we write nothing into one. The doctor issues a cross-border prescription in the format set out in the Annex to Directive 2012/52/EU, which reaches you as a PDF by e-mail. You print it or show it on screen at the pharmacy, together with a photo identity document. There is no code and no number to quote, and a pharmacy in Malta may dispense it; the decision rests with the pharmacist, who can refuse if the document is illegible or if its authenticity is in doubt.

If metformin has already been started for you and this is a continuation of established treatment, you can fill in the questionnaire at /repeat-prescription/. The review is carried out by lek. Damian Wojno, a doctor registered with the Polish Medical Chamber, licence no. 3211301; he is not registered with the Medical Council of Malta. A recent HbA1c and a recent measure of kidney function are the results the doctor asks for most often, because the dose depends on them. We do not start this treatment for the first time from a questionnaire, and we do not deal this way with the warning signs described above.

The assessment costs 24,90 €, with an optional priority surcharge of 9,90 €. The fee covers the medical consultation, not the issuing of a document, and it is not funded by the Maltese public health system. If the doctor declines on medical or legal grounds, we refund the fee. If the doctor asks for documents, you have at least seven days to send them in, and if the consultation cannot be completed, we close the request and refund the fee in full. The medicine itself is paid for separately at the pharmacy, and whether a product is government funded in Malta is decided under the Government Formulary List and the POYC scheme, not by us.