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Gallstones are thought to be caused by an imbalance in the chemical make-up of bile inside the gallbladder. Bile is a liquid produced by the liver to help digestion.

It's unclear what causes the chemical imbalance, but gallstones can form if there are unusually high levels of:

• cholesterol inside the gallbladder (about 4 out of 5 gallstones are made of cholesterol)
• a waste product called bilirubin inside the gallbladder (about 1 in 5 gallstones is made of bilirubin)

These chemical imbalances cause tiny crystals to develop in the bile.

These can gradually grow (often over many years) into solid stones that can be as small as a grain of sand or as large as a pebble.

Sometimes only 1 stone will form, but there are often several at the same time.

Gallstones are more common if you:

• are female, particularly if you have had children, are taking the combined pill, or are having high-dose oestrogen therapy
• are overweight or obese
• are aged 40 years or older (the older you are, the more likely you are to develop gallstones)
• have a condition that affects the flow of bile (such as cirrhosis, primary sclerosing cholangitis, or obstetric cholestasis)
• have Crohn's disease or irritable bowel syndrome (IBS)
• have a close family member who's also had gallstones
• have recently lost weight (from either dieting or weight loss surgery)
• are taking an antibiotic called ceftriaxone