Drug cabinet: Diuretics

Diuretic tablets

Diuretics are often given to people who have high blood pressure or who are retaining fluid. Doireann Maddock puts some common questions about them to Dr Nigel Rowell, a GP who is Primary Care Lead for the Northeast Cardiovascular Network and National Clinical Adviser to NHS Improvement.

The close relationship between the heart and kidneys means that, if you have heart and circulatory disease, you may also experience problems with your kidneys. For example, if you have a weak heart that isn’t pumping properly (as a result of a heart attack, say), its output fails. The kidneys will sense this but will think it is happening because you are losing blood. It acts by telling the body to retain more fluid. Diuretics break this cycle by getting the kidneys to reverse their decision and get rid of water, not retain it. This gives relief to the whole circulatory system.

Why have I been given a diuretic?

Either because you’re suffering from high blood pressure or because your body is retaining too much fluid. Diuretics are particularly useful in treating heart failure, a condition in which the body holds too much water and salt. Diuretics act on the kidneys and encourage them to release water in the form of urine. As urine contains salt as well as water, diuretics also increase the body’s salt excretion – consisting of sodium, potassium and magnesium. This loss of fluid is generally a good thing, but the loss of salts can occasionally cause side effects.

What are these side effects?

Muscle cramps, as a result of the body losing too much sodium, potassium or magnesium, can be a common problem. If you experience cramps when your salt levels are normal, then it’s likely the cause is an overall reduction in the volume of circulating fluid.

What are the different types of diuretic?

There are three main types of diuretic: loop diuretics, thiazide diuretics and potassium-sparing diuretics. Each works by affecting a different part of the kidneys.

"Diuretics are particularly useful in treating heart failure, a condition in which the body holds too much water"

Loop diuretics are commonly given for fluid retention (oedema), particularly in the lungs. They are very fast acting and can cause quite a lot of fluid loss. This can mean frequent visits to the toilet. If this becomes a problem in your day-to-day life, discuss it with your GP or nurse.

Sometimes the dose can be divided and spread out through the day, and adjustments can be made with timings to fit in with your commitments. Loop diuretics are shorter acting than other types of diuretic and the excess fluid is usually removed within six to eight hours. Common loop diuretics are furosemide and bumetanide. Torasemide is occasionally used.

Thiazide diuretics act within one to two hours of taking them and the effects can last for up to 24 hours. They are mainly prescribed at a low dose to help with high blood pressure. Commonly prescribed thiazides include bendroflumethiazide and indapamide. People may find that the effects of thiazide are milder than loop diuretics and water loss is less dramatic. Doctors usually prescribe potassium-sparing diuretics to treat the build-up of water in the body when it is associated with heart failure. If you have heart failure, you may be prescribed one of these and a loop diuretic.

Potassium-sparing diuretics are a weaker type of diuretic and increase the output of water, but prevent too much potassium being lost at the same time. This helps to maintain the balance of salts within the blood. Commonly prescribed potassium-sparing diuretics are spironolactone and eplerenone (these two are also known as aldosterone antagonists).

Are certain side effects more common with particular types of diuretic?

Loop diuretics are powerful and can cause you to lose too much fluid. This may cause dizziness when you stand up (postural hypotension). Taking thiazide or loop diuretics can cause a build-up of uric acid in the bloodstream that can then go into the joints (usually the foot) and trigger gout. For those who have no choice but to take diuretics, medication for gout such as allopurinol can help. In a small number of people, thiazide diuretics can cause an increase in blood sugar levels. We are not entirely sure why this happens but it is something that can be easily checked by your GP. If you have diabetes, you may need more of your usual medication to keep your blood sugars normal.

As with all side effects, your GP can diagnose and make appropriate adjustments to your medication.

It’s important that you let your GP know if you have an illness where you are vomiting, have diarrhoea or you are not drinking enough water. They may ask you to temporarily stop taking any diuretic tablets, so that you don’t become dehydrated.

Are there any foods that I shouldn’t eat?

If you’re taking a diuretic, it’s important not to have too much salt in your food because this counteracts the effect of the diuretic. It’s not possible to avoid salt completely because it’s in lots of food, to some degree. However, you should avoid high-salt foods, such as processed foods, and don’t add extra salt when cooking. Is there any medication that I should avoid?

"If you’re taking a diuretic, it’s important not to have too much salt in your food"

Soluble effervescent paracetamol and soluble effervescent co-codamol contain a lot of salt so should be avoided, and don’t take extra salt tablets or Andrews Liver Salts unless your doctor has said you can. Anti-inflammatory painkillers, such as ibuprofen and naproxen, can make the kidneys retain fluid and hamper their function, and should be avoided unless approved by your doctor.

The use of diclofenac (also an anti-inflammatory) is also linked to an increased risk of heart attack and stroke. Most people with established heart and circulatory disease are advised not to take diclofenac tablets. However, diclofenac in gel or cream form (topical) is OK to use.

Some patients with severe heart failure and fluid retention that is difficult to relieve may be prescribed a thiazide and a loop diuretic. However, this combination can cause profound fluid loss (diuresis) and is generally not advisable for most people.

Whatever combination of medication you are prescribed, you should continue taking them unless your GP or specialist has advised you otherwise.

Related publications

More useful information