Absorbing too much lidocaine can cause life-threatening side effects, including seizures, cardiac arrhythmias, and respiratory arrest.
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 Heating Pad Lidoderm Patch
Applying external heat—such as heating pads, electric blankets, or even sitting in a hot tub—increases blood flow to the skin and accelerates the absorption of lidocaine into your bloodstream. This can lead to: Absorbing too much lidocaine can cause life-threatening side
Because the patch numbs the skin, you may not feel if a heating pad is becoming hot enough to cause severe burns. Heating Pad Lidoderm Patch