Consuming spirulina may cause headaches, allergic reactions, muscle pain, sweating, and insomnia in some cases. People with allergies to seafood, seaweed, and other sea vegetables should avoid spirulina.

If you have a thyroid condition, an autoimmune disorder, gout, kidney stones, phenylketonuria (PKU), or are pregnant or nursing, spirulina may not be appropriate for you. You should check with your healthcare provider before taking it.

It's possible that spirulina grown in the wild can absorb toxins from water, such as microcystins (known to cause severe liver damage), pollutants, and heavy metals. Most spirulina sold in the United States is grown in laboratories.