The U.S. Food and Drug Administration (FDA) has issued a warning against two antiseizure medications that can cause a rare but potentially life-threatening reaction if not diagnosed and treated quickly.

Antiseizure drugs levetiracetam (sold under the brand names Keppra, Keppra XR, Elepsia XR and Spritam) and clobazam (sold under brand names Onfi and Sympazan) can cause a serious hypersensitivity reaction called drug reaction with eosinophilia and systemic symptoms (DRESS), the FDA said in a news release Tuesday.

The symptoms of DRESS typically appear within two to eight weeks after the introduction of the drug and may continue even after stopping it.

The signs include rash, fever, sore throat, swelling of the face, lymphadenopathy (swelling of lymph nodes), painful sores in the mouth, difficulty swallowing, yellowing of the skin, easy bruising, shortness of breath, fatigue and muscle pain.

Patients may exhibit a range of skin reactions, including hives, vesicles (small, fluid-filled sacs) and bullae (fluid-filled large blisters) on the skin.

If not treated, DRESS leads to complications that result in damage to the liver, kidneys, lungs, heart and pancreas. Patients may develop hepatitis, pneumonitis, myocarditis, pericarditis, nephritis or colitis. The mortality rate is estimated to be around 10%.

"It [DRESS] may start as a rash but can quickly progress, resulting in injury to internal organs, the need for hospitalization, and even death. As a result, we are requiring warnings about this risk to be added to the prescribing information and patient Medication Guides for these medicines," the news release read.

Officials found 32 cases of DRESS related to levetiracetam and 10 cases related to clobazam among children and adults worldwide. Most patients required hospitalization, and two patients treated with levetiracetam died.

Levetiracetam has been FDA-approved for 24 years. The antiseizure drug can be used independently or along with other medicines. Common side-effects include irritability, confusion, loss of balance or coordination, and extreme drowsiness.

Clobazam has been FDA-approved for 12 years. It is used to control seizures associated with Lennox-Gastaut syndrome (a severe form of epilepsy). It is used in combination with other medications. The side-effects associated with the drug include difficulty speaking or swallowing, tiredness, change in appetite and problems with muscle control or coordination.

FDA officials advise patients not to stop the antiseizure medications without discussing them with their healthcare professionals.

"Stopping these medicines suddenly can lead to uncontrolled seizures. It is important to seek immediate medical attention for DRESS. Patients who develop any unusual symptoms or reactions, including a rash, at any time while taking levetiracetam or clobazam should go to the emergency room immediately," they said.

Users and healthcare professionals can report any side-effect involving levetiracetam, clobazam or other medicines, to the FDA MedWatch program here.