Water kefir is a probiotic, just like commercially made yogurt, kombucha, etc.
It is best to start slowly – as it is powerful. Whenever you change the makeup of your internal ecosystem, there can be “die-off”. (If the new good bacteria are replacing some bad bacteria, Candida, etc.) Temporary symptoms of die-off include headache, general aches, nausea, diarrhea, etc., (basically flu-like symptoms) as your body tries to rid itself of toxins as quickly as possible, just like when you are ill. If your body does not contain many toxins, you may not experience this.
Start out by introducing water kefir slowly. Only 1/2 cup per day and increase gradually.