Kidney stones are usually formed following a build-up of a substance in the body.

This build-up may be any of the following:

• calcium
• ammonia
• uric acid (a waste product produced when the body breaks down food to use as energy)
• cystine (an amino acid that helps build protein)

Certain medical conditions can lead to an unusually high level of these substances in your urine.

You are also more likely to develop kidney stones if you do not drink enough fluids.

Recurrent kidney stones 

You are at a greater risk of developing recurrent (returning) kidney stones if:

• you eat a high-protein, low-fibre diet
• you are inactive or bed-bound
• kidney stones run in your family
• you have had several kidney or urinary infections
• you have had a kidney stone before, particularly if this was before you were 25
• only one of your kidneys works
• you have had an intestinal bypass (surgery on your digestive system), or a disease of the small intestine, such as Crohn's disease (inflammation of the gut)

Medication

There is evidence that certain types of medication may increase your risk of developing recurrent kidney stones. These include:

• aspirin 
• antacids 
• calcium and vitamin D supplements 
• diuretics (used to reduce fluid build-up)
• certain antibiotics
• certain antiretroviral drugs (used to treat HIV)
• certain anti-epileptic drugs (used to treat conditions such as epilepsy)

Types of kidney stones

Kidney stones can develop as a result of a number of different factors. The causes of the four main types of kidney stone are outlined below.

Calcium stones

Calcium stones are the most common type of kidney stone. They are caused when there is too much calcium in the urine. High amounts of calcium could be a result of:

• an inherited condition called hypercalcuria, which leads to large amounts of calcium in urine
• high levels of vitamin D
• an overactive parathyroid gland (your parathyroid glands help to regulate the amount of calcium in your body)
• kidney disease
• a rare disease called sarcoidosis 
• some cancers

Calcium stones are usually either large and smooth or spiky and rough.

Struvite stones

Struvite stones are often caused by infections, and they most commonly occur after a urinary tract infection that has lasted a long time.

Struvite stones are more common in women than in men.

Uric acid stones

Uric acid stones often form when there is a high amount of acid in your urine. Uric acid stones may be caused by:

• eating a high protein diet that includes lots of meat
• a condition such as gout that prevents the body breaking down certain chemicals 
• an inherited condition that causes higher levels of acid in the body
• chemotherapy (a treatment for cancer)

Cystine stones

Cystine stones are the rarest form of kidney stone. They are caused by an inherited condition called cystinuria, which affects the amount of acid that is passed in your urine.