Diuretics are medicines used to treat fluid retention. Your pet’s kidneys manage fluid levels in their body. If their body is dehydrated, the kidney reabsorbs water preserving as much as possible. If blood volume and blood pressure rise, the kidney loses water to normalise blood pressure. The mechanism for controlling fluid movement involves sodium and chloride ions. These ions move water with them so if they are reabsorbed, water is reabsorbed and similarly both are lost together. Diuretic drugs work on this system.

They act to encourage water loss by urination, stopping the reabsorption of sodium and chloride salts (ions) in the kidney. Different types of diuretics work on different parts of the kidneys and have different mechanisms. Other salts such as potassium, hydrogen and calcium ions may also be affected by diuretics.  

What are the symptoms of fluid retention?

Several conditions cause fluid retention. Whatever the cause, though, if untreated it can lead to high blood pressure and organ failure. Fluid retained in the body is called oedema – but the symptoms depend on where that fluid is retained.  

  • Oedema in the lungs or chest causes laboured, rapid breathing and coughing. A cat’s purr may alter with a bubbling fluid sound. There may be a clear discharge from the nose. This is very dangerous as it may lead to internal drowning, and is usually an emergency.
  • Oedema in the abdominal cavity is called ascites. In this condition the abdomen appears swollen and there may be rapid weight gain. 
  • Some conditions cause widespread oedema, the face and limbs can be affected. The areas involved appear swollen and feel puffy and soft when stroked. If you press the swollen area with your finger a mark will often remain, this is called pitting oedema. 

A pet can have oedema and seem perfectly well or be off colour with reduced appetite and energy. It depends how severe the fluid retention is, and where it is.

When are diuretics used?

Heart Disease

The heart is essentially a pump. If it is diseased, it fails to pump blood effectively around the body. Sensors in the kidney detect this fall in blood pressure. This causes the kidney cells to retain water to raise the blood pressure again. The heart cannot pump this extra fluid around the body, so oedema accumulates in the lungs and abdomen. This is called congestive heart failure.

A diuretic called furosemide is usually used to treat congestive heart failure. It is a loop diuretic and acts powerfully and quickly to remove fluid and encourage urination. Furosemide leads to potassium loss in the long term and is often used with a potassium sparing diuretic, such as spironolactone. Spironolactone is also useful in cardiac disease as it has a protective effect on heart muscle. In advanced heart failure a third diuretic may be added. Often a thiazide diuretic such as hydrochlorothiazide will be used. 

Kidney Disease

Diuretics may be used alongside intravenous fluid therapy in acute kidney disease. Acute kidney disease is usually caused by a toxin, inflammation or infection damaging the kidney. Fluids are used to rehydrate the body and remove the waste products which accumulate when the kidneys are not working. Diuretics may be used to speed the removal of the waste products by increasing urination. Mannitol, an osmotic diuretic, or furosemide are often used in this case. 

Nephrotic syndrome is a rare condition of the dog and cat resulting in protein loss by the kidneys. Low protein levels cause oedema as water will not stay in the blood vessels when protein levels are very low. Oedema forms throughout the body in this disease, furosemide is often used to treat the oedema.

Bladder disease

Cats can be affected by cystitis with no obvious cause. This is called idiopathic feline lower urinary tract disease, or FLUTD. It is a frustrating and painful condition where the bladder becomes inflamed. There can be a build-up of inflammatory chemicals Diuretics are sometimes used to flush out the bladder and reduce the inflammatory chemicals. 

Both dogs and cats can form calcium containing bladder stones (uroliths). These stones can be hard to dissolve by diet. So, thiazide diuretics can be used to promote loss of calcium in the urine and reduce formation of stones.

High blood pressure/hypertension

High blood pressure occurs in both dogs and cats. Primary hypertension occurs with no cause, secondary hypertension with another condition such as hyperthyroidism or kidney disease. A calcium channel blocking drug such as amlodipine is usually used to lower blood pressure. However, their use in the long term can cause the kidney to retain fluid. A diuretic is then added to the treatment protocol.

Liver disease

A damaged liver cannot process proteins normally, so protein levels often drop causing oedema. Fluid accumulates in the abdomen. If this fluid is drained, it refills quickly causing an abrupt drop in blood pressure. So, diuretics are usually used to reduce the ascites.

Head trauma

Brain bruising and swelling can occur in head trauma. This can be dangerous, causing seizures, paralysis and death. Mannitol is used to draw fluid from the cells and reduce brain swelling.


This condition occurs when fluid cannot drain from the eye. The pressure within the eye increases causing pain, damage to the eye and often blindness. Diuretics may be used to reduce the fluid within the eye.

Metabolic and Electrolyte issues

There are several conditions where diuretics can help to manage fluid and ion disturbances while investigation is carried out to find a diagnosis and treatment.

High blood potassium/Hyperkalaemia

Can occur with kidney disease, blocked or ruptured bladder. Diuretics can be used temporarily with intravenous fluids to reduce potassium levels and prevent heart damage, if the animal can urinate. 

High blood calcium/Hypercalcaemia

Cancer, kidney disease and hormone conditions can cause high blood calcium. High levels of calcium in the body can damage the kidneys so diuretics can be used to lower levels in the blood. 


Fluid accumulation can be seen in the abdomen with cancer. This may be solid tumours producing fluid or because of protein loss. Diuretics can help to reduce the amount of fluid making the animals more comfortable. 

Feline infectious peritonitis is a viral disease in the cat. It can be very severe and the wet form results in abdominal oedema as protein levels drop and inflammation occurs throughout the abdomen. 


Diuretics are effective medicines which can significantly improve an animal’s quality of life. It is essential that animals on diuretics have free access to water as they may become dehydrated otherwise.