Furosemide: uses, side effects and cautions

Furosemide belongs to the diuretic group. Its job is to push the kidneys to send extra salt and water out as urine, which lowers the volume of fluid in the body. The NHS describes two main reasons to prescribe it: swelling caused by fluid retention, known as oedema, and high blood pressure.

The swelling it treats is usually a symptom of something else, most often a heart, kidney or liver condition. That is why furosemide is typically one part of a wider plan rather than a stand-alone fix. A clinician decides the dose based on what is causing the fluid and how your kidneys are working.

It comes as tablets or as a liquid. The NHS says doctors tend to begin with a low dose and may raise it after a week or more. At first it is usually taken once a day, and if the dose goes up it may be split into two doses.

Timing matters because the medicine works through urine. Taking it in the morning, or in the morning and late afternoon if it is split, keeps trips to the toilet away from the night. Swallow tablets with water, with or without food. Check with your doctor how much fluid you are allowed to drink, as advice differs between conditions.

• Take it at the same time each day so the pattern becomes predictable.

• Plan the first few hours after a dose around somewhere you can reach a toilet easily.

• If you use the liquid, measure it with the device supplied rather than a kitchen spoon.

Most adults can take furosemide, and it is sometimes suitable for children. The NHS lists several situations where it may not be suitable: a previous allergic reaction to furosemide or another diuretic, kidney or liver problems, diabetes, Addison's disease, a history of gout and low blood pressure.

These are not all automatic bans, but each one changes the risk and may call for extra monitoring or a different medicine. Tell the prescriber about any of them, and about anything you take over the counter.

Because it removes fluid, the effects people notice first are passing more urine, feeling thirsty with a dry mouth, dizziness, headaches and muscle twitches. Rarely it can cause a serious allergic reaction, severe skin blistering known as Stevens-Johnson syndrome, kidney problems, or severe dehydration.

Warning signs of dehydration in the NHS guidance include peeing much less than usual, dark yellow or brown urine, dizziness, tiredness, sunken eyes and confusion. If you think you are having a serious reaction, use NHS 111, and call 999 for anaphylaxis, which shows as a swollen throat or tongue, a raised itchy rash and difficulty breathing. Do not drive or use machinery if you feel dizzy or drowsy.

• Common: more urine, thirst, dry mouth, dizziness, headache, muscle twitches.

• Rare but serious: anaphylaxis, blistering skin, kidney problems, severe dehydration.

The NHS names a long list of medicines that can interact with furosemide, including other blood pressure tablets such as beta blockers and ACE inhibitors, heart rhythm medicines such as digoxin and amiodarone, potassium supplements, anti-inflammatory painkillers, laxatives, fluconazole, some antidepressants and antipsychotics, and lithium. A pharmacist can check a full list for you.

Alcohol is best avoided because it can add to dizziness, light-headedness and dehydration. Furosemide is not usually used during pregnancy or breastfeeding, and if you are pregnant or planning a pregnancy you should speak to your doctor about alternatives.

If you forget a dose, take it when you remember unless the next one is nearly due, in which case skip it. Never take two doses together to catch up. If you take more than prescribed, contact NHS 111.

Long-term users are offered regular blood tests, mainly so that the effect on salts and kidney function can be tracked. Do not stop furosemide yourself, since the swelling it controls may return; speak with your prescriber or pharmacist if side effects are a problem and ask about other options.