Dantrolene is a muscle relaxer that is used to treat muscle spasticity (stiffness and spasms) caused by conditions such as a spinal cord injury, stroke, cerebral palsy, or multiple sclerosis.

Dantrolene is also used to treat or prevent muscle stiffness and spasms caused by malignant hyperthermia (a rapid rise in body temperature and severe muscle contractions) that can occur during surgery with certain types of anesthesia.

Dantrolene may also be used for purposes not listed in this medication guide.

You should not use dantrolene if you are allergic to it, or if you have:

• active liver disease such as hepatitis or cirrhosis.

Do not take dantrolene at a time when you need muscle tone for safe balance and movement during certain activities. In some situations, it may be dangerous for you to have reduced muscle tone.

You may be more likely to develop liver problems while using dantrolene if you are a woman, if you are older than 35, or if you also take other medications. Ask your doctor about your individual risk.

Tell your doctor if you have ever had:

• liver disease;
• heart disease or a heart attack; or
• a breathing disorder such as COPD (chronic obstructive pulmonary disease).

It is not known whether this medicine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.

Do not breastfeed while using this medicine, and for at least 3 days after your last dose.

Dantrolene should not be given to a child younger than 5 years old.

Use Ryanodex (Intravenous) exactly as directed on the label, or as prescribed by your doctor. Do not use in larger or smaller amounts or for longer than recommended.

Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Call your doctor right away if you have signs of liver problems--nausea, upper stomach pain, itching, tired feeling, loss of appetite, dark urine, clay-colored stools, or jaundice (yellowing of the skin or eyes). These side effects may be most likely to occur between 3 and 12 months of your treatment with dantrolene.

Also call your doctor right away if you have:

• severe drowsiness;
• severe muscle weakness;
• weak or shallow breathing;
• severe or ongoing vomiting or diarrhea;
• problems with vision or speech;
• painful or difficult urination;
• a seizure; or
• a light-headed feeling, like you might pass out.

Common side effects may include:

• dizziness, drowsiness;
• diarrhea;
• weakness; or
• feeling tired.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Avoid driving or hazardous activity until you know how this medicine will affect you. Dizziness or drowsiness can cause falls, accidents, or severe injuries.

Avoid exposure to sunlight or tanning beds. Dantrolene can make you sunburn more easily. Wear protective clothing and use sunscreen (SPF 30 or higher) when you are outdoors.