Because the patch numbs the skin, you may not feel if a heating pad is becoming hot enough to cause severe burns.
Absorbing too much lidocaine can cause life-threatening side effects, including seizures, cardiac arrhythmias, and respiratory arrest. Heating Pad Lidoderm Patch
The medical consensus and official FDA labeling for Lidoderm (5% lidocaine patch) strongly advise using a heating pad directly over the patch . Why Heat is Dangerous with Lidoderm Because the patch numbs the skin, you may