Loop diuretics

Diuretics

A diuretic is a medicine which increases the amount of urine that you pass out from your kidneys. A diuretic causes an increase in urine (a diuresis). So, they are sometimes called 'water' tablets. There are three main types of diuretic:

• Loop diuretics (for example, furosemide).

• Thiazide diuretics (for example, bendroflumethiazide). These are mainly used for treating high blood pressure (hypertension) and fluid on the legs (oedema).

• Potassium-sparing diuretics. Potassium-sparing diuretics are weak diuretics usually prescribed in combination with other types of diuretics. They are used to increase the amount of fluid passed from the body in urine, whilst also preventing too much potassium being lost with the urine.

They work by making the kidneys pass out more fluid. They do this by interfering with the transport of salt and water across certain cells in the kidneys. (These cells are in a structure called the loop of Henle - hence the name loop diuretic. You have thousands of these loops in each kidney.) As more fluid is passed out by the kidneys, less fluid remains in the bloodstream. So any fluid which has accumulated in the tissues of the lungs or body is drawn back into the bloodstream to replace the fluid passed out by the kidneys. This eases symptoms such as oedema and breathlessness caused by the congestion of fluid.

Furosemide and bumetanide are both loop diuretics and are similar in activity. Both start to work within 1 hour of being taken by mouth and diuresis is complete within 6 hours so that, if necessary, they can be given twice in one day without interfering with sleep. When given into a vein, furosemide has a peak effect within 30 minutes. Furosemide and bumetanide are mainly used to treat heart failure, including acute pulmonary oedema.

Torasemide has properties similar to furosemide and bumetanide, and is used to treat oedema and for high blood pressure (hypertension).

Side-effects are uncommon when routine low doses are used. The higher the dose, the greater the risk of side-effects developing. The leaflet which comes in the tablet package provides a full list of possible side-effects. So, it is important to read that leaflet if you are prescribed a loop diuretic. Common or serious possible side-effects include:

• The salt balance in the bloodstream sometimes being upset, which can cause a low blood level of potassium, sodium, and magnesium, and a high level of calcium. These effects may cause weakness, confusion and, rarely, abnormal heart rhythms to develop. You may be advised to have a blood test to check for these problems.

• If you have diabetes or gout, these conditions may be made worse by diuretics.

• An upset stomach.

• Dizziness on standing - due to too low blood pressure (hypotension).

Most loop diuretics are taken once a day in the morning. The effect of making more urine usually starts within half an hour. So, you may find you need to go to the toilet a couple of times within a few hours of taking the tablet. However, the effect of passing extra urine wears off within about six hours. So, you will not have to get up in the night to make extra trips to the toilet.

You can generally take the dose at a flexible time to suit you. So, for example, if you want to go out early in the morning for a few hours and don't want to have to find a toilet, you may be able to postpone the dose until later on in the day when it may be more convenient to find toilets.