"Drink eight glasses of water a day to flush your kidneys" is one of the most repeated pieces of health advice in Britain. It is also, mostly, a myth.
What the NHS actually says
NHS Eatwell guidance recommends 6 to 8 glasses of fluid a day for adults — and that includes water, lower-fat milk, sugar-free drinks, tea and coffee. There is no medical magic in plain water.
Does extra water "flush" the kidneys?
For people with healthy kidneys, drinking more than you need does not improve kidney function. It just means more trips to the loo. Research summarised by the UK Kidney Association has not shown that high fluid intake slows progression of CKD in the general population.
When more fluid genuinely helps
- Kidney stones: NICE recommends drinking 2.5–3 litres a day to reduce recurrence.
- Urinary tract infections: Staying well hydrated supports recovery.
- Hot weather, exercise, illness: Increased losses need replacing.
When less fluid is appropriate
People with advanced CKD, those on dialysis, or those with heart failure may be advised by their consultant to restrict fluids. Always follow your renal team's individualised target rather than generic advice.
How to check you are well hydrated
The simplest measure: your urine should be pale straw colour. Very dark urine usually means you need to drink more; very pale, frequent urine usually means you can ease off.