Similar to EMS, TENS side effects often happen as a result of improper use. Some users may inadvertently set the intensity too high, or the pads were incorrectly attached, resulting in discomfort. Other people may also be allergic to the sticky pads. Leaving the electrode stuck to your skin for a long time can lead to redness or skin irritation. If this happens, you can switch to machines with hypoallergenic pads instead.
Experts recommend using TENS for 30 minutes while being active. Still, consult your doctor to determine how often your sessions should be. Follow the instructions for proper use, too. Also, keep in mind that a TENS device is not a cure for persistent muscle pain. Overusing it might only lead to serious injury.