Causes

Doctors believe that gallstones develop because of a chemical imbalance in the bile inside the gallbladder. Bile is a liquid. It's produced by the liver to help digestion.

It's still unclear exactly what leads to this imbalance. But gallstones can form if there are unusually high levels of:

• cholesterol inside the gallbladder
• bilirubin inside the gallbladder

Bilirubin is a waste product. About 1 in every 5 gallstones is made up of bilirubin.

About 4 in every 5 gallstones are made up of cholesterol.

These chemical imbalances cause tiny crystals to develop in the bile. These can grow into solid stones. This often happens over many years. The stones 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.

At risk groups

Gallstones are more common if you are female, particularly if you:

• have had children
• are taking the combined pill
• are undergoing high-dose oestrogen therapy

They are also more common if you:

• are overweight or obese
• are aged 40 years or older - the older you are, the more likely you are to develop gallstones
• 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
• have a condition that affects the flow of bile

Conditions that affect the flow of bile include:

• cirrhosis (scarring of the liver)
• primary sclerosing cholangitis
• obstetric cholestasis