Swollen knee

A swollen knee means there's a fluid on the knee. It’s sometimes called knee effusion or water on the knee.

By Wallace Health I Medically reviewed by Adrian Roberts.
Page last reviewed: October 2018 I Next review due: October 2023


Knees are very prone to swelling and pain because they're weight-bearing. The swelling occurs when fluid builds up in or around your knee joint. This is sometimes called water on the knee but is medically called an effusion. The most common causes are:

  • Injury or trauma
  • Overuse
  • Underlying disease

Symptoms of a swollen knee

There are several different causes of knee swelling: 

  • Effusion — when fluid builds up in the knee joint
  • Haemarthrosis — when blood builds up in the knee joint
  • Oedema — when fluid builds up in the tissues around the knee joint

Swelling of the knee can cause these symptoms:

  • A red and warm knee
  • A tender knee
  • Difficulty bending or straightening your knee completely due to stiffness
  • Inability to put weight on your knee due to pain
  • Occasionally, ‘locking’ of the knee joint 
  • Puffy skin around your kneecap  

Knee pain is also common. The onset of swelling and the type of pain varies depending on the cause. Inflammation, which causes swelling, can occur suddenly (acute) or gradually and last for a long time (chronic). Acute swelling is often caused by an injury and will disappear after a day. Chronic swelling can be a sign of an underlying medical condition.

In many cases, swelling can be treated at home with painkillers and by elevating your leg. However, you should see your GP if you think your knee is infected, or if your swelling doesn’t go down after a few days. Signs of an infection are:

  • Hot to touch
  • Rapid onset of swelling
  • Tenderness and pain

Causes of swollen knee

If your pain and swelling have come on suddenly, it may be due to an injury from playing sport or after an accident or fall. Common injuries include:

If your swelling has come on gradually without any obvious trigger, it could be due to overuse of the knee joint. This often happens if you play a lot of sport or through repeated bending or kneeling. Conditions include:

  • Bursitis knee (sometimes called housemaid’s knee) — inflammation of jelly-like sacs (bursae) that cushion your knee joint, which causes swelling and pain; to diagnose bursitis your doctor will physically examine your knee and may recommend you have an X-ray or MRI scan; after diagnosis, treatments include: 
    • Compression of the knee with an elastic bandage
    • Elevating the knee
    • Periodic application of ice packs
    • Rest
    • Taking nonsteroidal anti-inflammatory drugs (NSAIDs) eg ibuprofen to relieve your pain
  • Tendinopathy

Chronic swelling, often with night-time pain, could be due to an underlying disease. These conditions, such as arthritis, are more likely as you age.

If you have arthritis, you will have chronic inflammation but you may also have symptoms of acute inflammation. There are different types of arthritis, which can all cause a swollen knee. These include: 

  • Gout — caused by too much uric acid in your blood
  • Infectious arthritis — caused by an infection of a joint that leads to swelling
  • Juvenile arthritis — occurs in people aged up to 16 years
  • Osteoarthritis — the most common form of arthritis caused by break-down of cartilage in the joints
  • Reactive arthritis — usually occurs after an infection of your genitals or urinary tract and becomes chronic; this is not common
  • Rheumatoid arthritis — your immune system mistakenly attacks your joints

If your doctor thinks your swollen knee could be caused by arthritis, they will ask you about any other symptoms and your medical history. They may refer you for further investigations, such as blood tests or an X-ray. 

Treatments for arthritis vary depending on the type you have. They include: 

  • Lifestyle changes eg exercise
  • Medications — this includes:
    • Corticosteroids
    • Disease-modifying anti-rheumatic drugs
    • NSAIDs to relieve your pain
  • Physiotherapy
  • Self-help devices eg easy-grip utensils, jar openers and reachers
  • Surgery

Many of these knee problems and injuries can also cause a Baker’s cyst. This is when joint fluid leaks out into the back of the knee and causes it to swell.

Less common causes of a swollen knee include:

  • An infection — this includes: 
    • Lupus — an autoimmune disease that can be triggered by an infection and causes your immune system to mistakenly attack healthy tissues, including your joints
    • Lyme disease — caused by bites from ticks infected with bacteria, which causes a rash, flu-like symptoms and joint swelling
  • Bleeding in the joint (haemarthrosis) caused by an injury — this needs urgent medical treatment
  • Reactive arthritis
  • Septic arthritis
  • Tumours

Complications of a swollen knee

You may develop a Baker's cyst. This is when joint fluid leaks out into the back of the knee and causes pain and swelling. Treatment usually involves compression and applying ice packs. However, if your swelling is severe, you may need to have the fluid removed using a fine needle.

You may lose muscle mass, especially in your thigh muscles. This is because fluid in your swollen knee can prevent your thigh muscles from working properly; over time this causes them to weaken and deteriorate.

Risk factors for a swollen knee

  • Being overweight or obese — your knees are weight-bearing so any excess weight puts more strain on them, which can damage your knee joint over time; obesity also increases the risk of osteoarthritis, which is a common cause of a swollen knees
  • Playing certain sports — if you take part in sports that involve pivoting, rotating or twisting your knees, you’re at greater risk of knee injuries, which cause swelling
  • Your age — your risk increases as you get older

Talk to your doctor if you’re concerned about symptoms

Book an appointment with a Spire GP today

Getting a diagnosis for swollen knee

Chronic swelling can cause permanent damage to the joint tissue, cartilage and bone. It is therefore important to ask your doctor for advice if your swelling doesn’t go down.

They'll discuss your symptoms and carry out a physical examination.

They may arrange for you to have some tests. These may include an:

They may also arrange for you to have joint aspiration. This is when a fine needle is inserted into the swollen area to check for blood, bacteria or crystals (which are often found in people who have gout or pseudogout)

Treatments for swollen knee

  • Your treatment will depend on what’s causing your swollen knee and how painful it is. Your doctor will recommend the most appropriate treatment after diagnosing the underlying cause. 

    In most cases, you’ll be advised to take painkillers. You can also apply ice and elevate your knee to help reduce the swelling. Sometimes removing some of the fluid in your knee (joint aspiration) helps reduce the pain and stiffness. 

    Other treatments include:

How to prevent a swollen knee

A swollen knee is usually caused by an injury or an underlying medical condition. To reduce your risk of injuries and maintain your health, you can: 

  • Stay a healthy weight — excess weight puts pressure on your knee joints and increases wear and tear over time
  • Strengthen the muscles around your knee — this will reduce the pressure on your knee joints
  • Try low-impact exercises eg swimming or water aerobics — these will help your body stay healthy without placing continuous or excessive weight on your knee joints


Frequently asked questions

When should I worry about a swollen knee?

If your knee pain is severe or your swelling has not reduced after a few days, you should see your GP. You should also see your GP if you’re concerned that your knee is infected — signs of infection include your knee suddenly becoming swollen, and feeling hot to touch, painful and tender.

How long does a swollen knee last?

This depends on the underlying cause. If the swelling came on suddenly due to an injury, it may only last a day.

If the swelling doesn’t go down after a few days, you should see your GP. You may need treatment to address the cause of your swelling eg due to an underlying disease, infection or more serious injury.

Can a swollen knee go away on its own?

Yes, a swollen knee can go away on its own, especially if it came on suddenly after an injury. It may disappear after a day. However, if your swelling doesn’t disappear after a few days, you should see your GP for a diagnosis and potential treatment.

Does fluid in the knee go away?

Fluid in your knee is usually caused by inflammation, resulting from an injury or underlying health condition. Depending on the cause, it can go away on its own. However, if it does not go away after a few days, you may need treatment and should see your GP.

What is the best anti-inflammatory for knee pain?

Knee pain can be effectively reduced by taking non-steroidal anti-inflammatories (NSAIDs) such as ibuprofen. 

Should you walk with a swollen knee?

This depends on the cause of your swollen knee and whether you have knee pain eg if you have mild to moderate knee pain due to osteoarthritis, walking can help strengthen your muscles and so reduce your knee pain and swelling. However, if you have severe knee pain due to an injury, you may need to rest your knee.

To find out what level of movement is appropriate, speak to your GP. They can advise you based on the underlying cause of your swollen knee and your pain levels.