Water on the knee: what a swollen knee means and what to do

Reviewed by The Wellness Team. Last reviewed October 2026.

Short answer: "water on the knee" is the everyday name for a swollen knee. The swelling is a sign that something is irritating the joint. It is not a diagnosis in itself, so the useful question is what is causing it. A swollen knee that is also hot, red or comes with a fever needs urgent advice, not home care.

What water on the knee means

Fluid collects in and around the joint when the knee is injured, overused or inflamed. Common reasons include a sprain or other injury, arthritis flaring up, and overuse. The swelling is the knee's response. Treating the cause matters more than draining or ignoring the fluid.

What the NHS says to do first

For knee pain and swelling the NHS advises:

• Put as little weight as possible on the knee, and avoid standing for a long time.
• Use an ice pack or a bag of frozen peas wrapped in a tea towel for up to 20 minutes every 2 to 3 hours.
• Use painkillers such as paracetamol, or ibuprofen gel or tablets.

See a GP if knee pain does not improve within a few weeks. In many areas you can also refer yourself to an NHS community musculoskeletal (MSK) service for physiotherapy without seeing a GP first.

When to get urgent advice

The NHS says to call 111 now if:

• Your knee is very painful, or badly swollen or has changed shape.
• You cannot move the knee or put any weight on it.
• The knee locks, gives way or painfully clicks. Painless clicking is normal.
• You have a very high temperature, or feel hot, cold or shivery, and the knee is red or hot. This can be a sign of infection.

If it keeps coming back

Swelling that returns after activity, with stiffness or aching, can point to osteoarthritis or a worn cartilage or meniscus problem. NICE guideline NG226 puts exercise, strengthening and weight management first for osteoarthritis. A scan is not always needed to start those. If the swelling keeps recurring, it is worth an assessment so the cause is named rather than guessed.

Where PRP fits

Platelet-rich plasma (PRP) is made from a small sample of your own blood and injected into the joint. It is not a treatment for an acute swollen knee, and you should not have an injection into a hot, red or newly injured joint. For a knee with long-standing, diagnosed osteoarthritis or a tendon problem, PRP can be one option to discuss after the first-line steps. The research is mixed and results vary. We do not guarantee an outcome.

Our prices are plain: PRP for a joint is £750.

Next step

If your knee swells repeatedly and you want an honest view of the cause, we can talk through your history and tell you whether an injection makes sense or whether another route comes first.

Message us on WhatsApp or read our knee injection guide.

Sources: NHS, Knee pain (nhs.uk/symptoms/knee-pain); NICE guideline NG226, Osteoarthritis in over 16s. This article is general information and not a diagnosis.