Spirulina is a blue-green algae sold as a powder or tablet. It supplies a lot of protein, plus B vitamins, iron, and antioxidants like phycocyanin. People stir the powder into smoothies, juice, or water, or take tablets with food for general nutrition and immune support.
Evidence that spirulina eases oxidative stress or boosts immune cell activity is thin. No specific health outcome has a strong body of human evidence behind it. The supplement is food, not medicine.
Cautions: Poorly grown spirulina can carry heavy metals or microcystins. Buy only from makers who publish purity testing. Skip it if you have phenylketonuria (PKU) because spirulina contains phenylalanine. Because it may rev up the immune system, talk to a doctor before using it if you have an autoimmune condition or take immunosuppressant drugs.