There are a number of reasons one would experience shocks, and a number of ways to avoid it. Please check this blog for details:
The maximum current from the colloidal silver output is 2mA. This is under short-circuit conditions, so the current during treatment will be less. We do not know why your body is able to sense current this low. Perhaps you are very sensitive.
Ensure your tens cables are not damaged. They need to be made stronger as they are flimsy.
Ensure you're using a suitable Contact preset. I feel this too needs to be made clear before use.
For more details, please check the link: