Metformin is an oral blood glucose lowering drug used to control type 2 diabetes. It is a member of a group of drugs known as biguanides, which has been in use for many years. Metformin is known to be a safe and effective form of treatment.
Metformin works by reducing the amount of glucose produced by the liver and increasing the uptake of glucose by the cells of the body. This has the overall benefit of reducing blood glucose levels by 3-4 mmol/l.It does not increase how much insulin is made by the pancreas unlike other drugs such as Gliclazide.
Metformin is also thought to help by lowering component of blood fats, that are often high in people with type 2 diabetes. It also decreases your appetite which helps you lose a few pounds in weight.
The most difficult problem with Metformin is due to its effect on the gastrointestinal system. It may cause a wide range of effects from a mild loss of appetite to nausea, vomiting, abdominal discomfort, cramps, flatulence and diarrhoea. Some people describe a temporary unpleasant or metallic taste when they start taking Metformin though this is unusual.
If you do experience stomach upset, it is recommended that you try to persevere with Metformin, even if you find you can only tolerate a small dose of one or two tablets daily.
Some find these symptoms impossible to cope with and discontinue the tablets within days. Others find the effects tolerable and find that things improve with time. Starting off with a small dose taken at meal times helps.
Hypoglycaemia is not usually a problem associated with Metformin.
Lactic acidosis, is a rare but dangerous side effect of Metformin. This is a serious condition where the cells of the body do not get enough oxygen to survive. It is caused by a build up of lactic acid in the blood. Most of the cases described have been in people whose kidneys were not working normally. Although rare, if lactic acidosis does occur, it may be fatal in up to half the cases.
Your doctor will monitor your kidney function and carry out blood tests from time to time. There is no evidence that Metformin causes any damage to your kidneys or liver.
If you have an illness that results in severe vomiting and diarrhoea and if your intake of fluids is reduced, it may be necessary to stop taking Metformin temporarily. It is also usual to stop Metformin if you develop a serious condition such as a heart attack, stroke or severe infection.
If you are going to have surgery or have specialised x-ray procedures that require injection of contrast agents, Metformin should be stopped temporarily.
Ideally you stop oral medications before planning your pregnancy. If pregnancy is confirmed unexpectedly you should stop taking Metformin as with other oral hypoglycaemic medications. Your doctor will probably change you over to insulin.
Metformin is not recommended for the people with the following conditions:
The usual starting dose of Metformin is 500mg twice daily. Take the tablets with food or just before or after a meal. This may be increased by your doctor to 500 mg three times daily up to a maximum dose of 6 tablets (3 gm) each day.
As mentioned before, it is useful to start with just one tablet each day until your stomach gets accustomed to your new tablets.
Yes, Metformin can be combined with other oral blood glucose lowering tablets such as sulphonylureas and thiazolidenediones (Glitazones). It is also increasingly used in combination with insulin in people with type 2 diabetes.
Metformin is usually prescribed a ‘generic’ Metformin which is available in strengths of 500mg and 850 mg tablets. It is also now available in ‘slow release’ form as Glucophage SR 500mg. This preparation is discussed in a separate document.
Dr Nishan Wijenaike, MD FRCP
West Suffolk Hospitals Diabetes Service
First published online October 2002
Updated November 2007