They have a built in timer that is auto set to 30mins and limited to 60mins. Of course anything can be abused but, aside from making yourself sore they are pretty safe. If you imagined getting a strong massage in one spot for too long - a similar result.
You need to get medical advice if you're pregnant or epileptic and not to be used if you have a pacemaker or defibrillator fitted or other electronic device fitted to your body.
We have all ages using them including the elderly. They are really simple to use
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