The side effects of spirulina include sweating, trouble concentrating, headaches, muscle pain and flushing of the face. Additional side effects, such as liver damage, diarrhea and vomiting; are associated with certain types of spirulina that can be contaminated with heavy metals. People with allergies to blue-green algae or people who have phenylketonuria (PKU) are at a greater risk of an unpleasant reaction when taking spirulina.
Additionally, spirulina can interact with a number of different types of medications and herbal supplements. Speak to your doctor before taking spirulina if you are taking other medications or supplements for any of the following medical conditions: immune system disorders, inflammation, high blood pressure, high cholesterol, viruses, allergies, neurologic conditions, diabetes, weight loss, heart problems, osteoporosis, cancer and blood clots.
Spirulina's risk to pregnant women and their fetuses is not known at this time. If you are pregnant, you should speak to your doctor prior to taking spirulina.