Gallbladder sludge is a thick, viscous mixture of cholesterol crystals, calcium salts, and mucus that can accumulate in the gallbladder when bile sits too long without being emptied properly. It is essentially a halfway stage between normal bile and gallstones: the concentrated sediment may cause biliary colic — cramping pain under the right ribs after fatty meals — nausea, and inflammation, or it may cause no symptoms at all and be found incidentally on ultrasound. Prolonged fasting, rapid weight loss, pregnancy, and certain medications encourage its formation. Sludge often clears on its own once normal gallbladder emptying resumes, but persistent cases may progress to stones requiring medical management.