Depends what frequency your zapper runs on. With 15hz that will be the strongest, kind of a strong buzzy feeling, not painful though. 2500hz more of a high pitched buzz. 30,000hz is a very slight tingle, some feel nothing at all though. Go slow your first time. If you overdo it you may have some detox symptoms, headache, nausea, achy flu like etc. That's not a bad thing just give your body time to deal with it before zapping again if that happens. My zapper has been a real god send. Every time I feel like I'm on the verge of getting sick it pulls me out of it and usually feel better the next day.