The cause of leg cramps is sometimes unknown (idiopathic). In other cases, there may be an underlying condition or another identifiable cause.

Idiopathic leg cramps

Although the cause of idiopathic leg cramps is unknown, there are a number of theories about what might cause idiopathic leg cramps. These include:

• abnormal nerve activity during sleep which causes the muscle of the leg to cramp
• excessive strain placed on leg muscles, such as when exercising, may cause the muscles to cramp at certain times
• a sudden restriction in the blood supply to the affected muscles

Also, tendons naturally shorten over time as a person gets older, which may explain why older people are particularly affected by leg cramps. Tendons are tough bands of tissue that connect muscles to bone. If your tendons become too short, they may cause the muscles connected to them to cramp.

Secondary leg cramps

Secondary leg cramps are caused by an underlying condition or another identifiable cause. These include:

• pregnancy: the extra weight of pregnancy can place strain on the leg muscles, making them more vulnerable to cramping
• exercise: leg cramps are often experienced when resting after exercise
• neurological conditions (conditions that affect the nerves in your leg muscles): for example, motor neurone disease or peripheral neuropathy
• liver disease: if your liver stops working properly, toxins will build up in your blood, which can make your muscles go into spasm
• infection: some types of bacterial infection, such as tetanus, can cause muscle cramps and spasm
• toxins: in some people, high levels of toxic (poisonous) substances in the blood, such as lead or mercury, can cause leg cramps
• dehydration: in some people, low levels of water in the body can lead to a drop in your salt levels, which can trigger muscle cramps

Medication

Certain medications have been known to cause leg cramps in a small number of people. These include:

• diuretics: these remove fluid from the body and are used to treat conditions such as high blood pressure, heart failure and some types of kidney disease
• statins: these are used to treat people with high cholesterol levels in their blood 
• raloxifene: this is used to prevent osteoporosis (thinning of the bones) in women who have experienced the menopause
• nifedipine: this is used to treat angina and Raynaud’s phenomenon 
• nicotinic acid: this is used to treat high cholesterol

Contact your GP if you think your medication may be causing your leg cramps as your dosage may need to be adjusted. Never stop taking a prescribed medication unless your GP or another qualified healthcare professional who is responsible for your care advises you to do so.