There are no known side effects of using tapping. With that being said, you should keep four things in mind.
1) Before you begin any physical or emotional treatment you should ALWAYS seek the advice of your primary care physician. This is true for a new diet, a new exercise program, or tapping. The understanding of the human body is constantly growing and changing. Make sure you find the most current information before making any decisions about the type of care you choose.
2) If you encounter any negative side effects or adverse reaction, STOP. Seek the advice of a trained medical professional. Note that it is common for the tapping points to be a little sensitive. The article It Hurts! discusses what you can do when tapping points feel tender.
3) Just because tapping is a useful tool for a problem, doesn’t mean you are the right person to use that tool. For example, tapping works wonderfully on Dissociative Identity Disorder (formerly known as multi-personality disorder), but there is no way that I am qualified to do that sort of work. DID is very complicated and I could do more harm than good.
So being proficient with tapping doesn’t mean you should use it with someone else to deal with an issue for which you aren’t qualified. Tapping is a tool to use in an appropriate context. If you are not trained to work in a certain context without tapping, then you shouldn’t use tapping in that context. You should seek the help of a trained professional.
4) Scratches and pimples are two physical reactions I have encountered using tapping. When my fingernails are too long I scratch myself when tapping. I also sometimes forget to wash my face after an hour of tapping. It’s easy to find the tapping points on my face the next day, just look for the pimples!