Almost every website says the following:
Note to beginners: Everyone responds to Spirulinaís cleansing properties differently, depending on current health and dietary history. Occasionally, Spirulina may cause discomfort to the first time user. Signs of cleansing may include mild headache, upset stomach, constipation, or diarrhea. Such cleansing responses indicate a very positive process. To avoid or reduce possible discomfort, begin taking 1/2 of the suggested use and increase slowly as your body adjusts to this potent nutrition.