Learned a New Thing
Why do NSAID's cause fluid retention?
NSAID's block the formation of certain "bad" prostaglandins which cause inflammation and pain. However, they also block some "good" prostaglandins too .. in particular those that are needed do keep blood vessels in the kidneys dilated. When blood vessels in the kidneys are constricted, then the flow of blood is reduced, which in turn reduces the glomerular filtration rate (GFR), or the rate at which the kidneys filter blood. This slowed filtration rate causes a bit of a backlog in the bloodstream which leads to increased blood pressure, so the body relieves the pressure by causing some of the excess water-fluid to seep through the blood vessel walls and into the tissues. The reduced filtration rate also causes the kidneys to retain sodium and potassium.
For most people, this effect is temporary and transient; it clears up once the NSAID is out of the system. However, persons with kidney disease, the elderly and also liver disease need to use these with caution, or even not at all. Also persons taking ACE-inhibitors for blood pressure, and potassium-sparing diuretics such as spironolactone should not use NSAID's without discussing with their dr. This includes aspirin!