According to MedlinePlus, some spirulina and other forms of blue-green algae may be contaminated by bacteria or heavy metals. In these cases, there is no safe dosage, as any amount may lead to serious health problems such as upset stomach, weakness, rapid heartbeat and liver damage. Contaminated algae may sometimes even be fatal. Spirulina grown in natural environments is most likely to be contaminated, so stick with varieties grown in controlled settings, and research the brand before you buy.