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People who get a leg cramp feel a sudden pain in their leg, often at night.

This happens when a muscle or group of muscles suddenly tightens (contracts).¹ This is called a muscle spasm. Most cramps happen in calf muscles, but you can also get cramps in your thigh or in your foot.

Leg cramps usually happen at night or when you are resting. The muscle contractions can last for a few seconds or up to 10 minutes.² You won't know how long they'll last.²

We don't know very much about what causes leg cramps or why certain people get them. Most people who have these pains in their legs are not sick. But the following things can make you more likely to get painful cramps:^{1 2}

• Being pregnant
• Being middle-aged or older
• Exercising
• Not having certain salts in your body
• Having dialysis (this is when a machine is used to clean your blood because your kidneys aren't working properly)
• Having an imbalance of minerals in your body (this is called an electrolyte imbalance)
• Having problems with your blood circulation or having varicose veins in your legs (varicose veins happen when valves in veins don't work properly, so blood pools in the veins)
• Having damaged nerves or a disease that affects your nerves
• Having a disease that affects your muscles.

Some medications can cause leg cramps as a side effect. They include:³

• Diuretics
• Nifedipine
• Beta-agonists
• Steroids
• Morphine
• Cimetidine
• Penicillamine
• Statins
• Lithium.

You can also get severe calf pain after an injury or if you have a Baker's cyst that bursts (a Baker's cyst is a lump of fluid that forms behind the knee).

A blood clot in your leg also can cause leg pain. This is known as deep vein thrombosis (DVT) and it is more serious than a leg cramp. You will need medication to treat the clot and keep it from causing serious problems. If you get swelling as well as pain in your calf or thigh, you might need an ultrasound scan to see if there's a blood clot in your leg. To learn more, see our articles on Deep vein thrombosis.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.