Eculizumab, sold under the brand name Soliris among others, is a recombinant humanized monoclonal antibody used to treat paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, generalized myasthenia gravis, and neuromyelitis optica.

[16] This binding prevents the breakdown of red blood cells in the bloodstream (intravascular hemolysis) in people with paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome.

[16] The most frequently reported adverse reactions for people with paroxysmal nocturnal hemoglobinuria include headache, nasopharyngitis (common cold), back pain and nausea[16] The most frequently reported adverse reactions for people with atypical hemolytic uremic syndrome include headache, diarrhea, hypertension, upper respiratory infection, abdominal pain, vomiting, nasopharyngitis, anemia, cough, swelling of lower legs or hands, nausea, urinary tract infections and fever[16] Eculizumab (Soliris) is developed, manufactured, and marketed by Alexion Pharmaceuticals.

[16] Eculizumab is used to treat atypical hemolytic uremic syndrome (aHUS) and paroxysmal nocturnal hemoglobinuria (PNH).

[8][7][15] For people with paroxysmal nocturnal hemoglobinuria, it improves quality of life and decreases the need for blood transfusions but does not appear to affect the risk of death.

[11] It does not appear to change the risk of blood clots, myelodysplastic syndrome, acute myelogenous leukemia, or aplastic anemia.

[7] Common adverse effects (occurring in between 1% and 10% of people who take the drug) include infections (pneumonia, upper respiratory tract infection, colds, and urinary tract infection), loss of white blood cells, loss of red blood cells, anaphylactic reaction, hypersensitivity reaction, loss of appetite, mood changes like depression and anxiety, a sense of tingling or numbness, blurred vision, vertigo, ringing in the ears, heart palpitations, high blood pressure, low blood pressure, vascular damage, peritonitis, constipation, upset stomach, swollen belly, itchy skin, increased sweating, blotches from small bleeds under the skin and skin redness, hives, muscle spasms, bone pain, back pain, neck pain, swollen joints, kidney damage, painful urination, spontaneous erections, general edema, chest pain, weakness, pain at the infusion site, and elevated transaminases.

Both C5a and C5b-9 cause the complement-mediated events that are characteristic of paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome.

[21] The metabolism of eculizumab is thought to occur via lysosomal enzymes that cleave the antibody to generate small peptides and amino acids.

[14] Eculizumab was approved by the US Food and Drug Administration (FDA) in March 2007, for the treatment of paroxysmal nocturnal hemoglobinuria.

[17]: 6 When the FDA approved it in September 2011, for the treatment of atypical hemolytic uremic syndrome, it designated it as an orphan drug.

[25] Eculizumab was approved by the FDA for the treatment of adults with neuromyelitis optica spectrum disorder who are anti-aquaporin-4 (AQP4) antibody positive in 2019, based on the results of the PREVENT trial.

[31][32] In April and May 2013, a controversy arose in Belgium when the media revealed that the government had refused to pay for a seven-year-old boy's treatment because Soliris was too expensive.

[38][39] In December 2013, New Zealand's government pharmaceutical buyer Pharmac declined a proposal to subsidize the drug after Alexion refused to budge on a NZ$670,000 (US$590,000) per person per year price and Pharmac's economic analysis determined the price would need to be halved before the drug was cost-effective enough to subsidize.

People can apply for it on "compassionate grounds" "on a case-by-case basis for example individuals who have been urgently hospitalized due to an immediate life-, limb-, or organ-threatening complication."

It then was already "funded by the Ontario government for the treatment of another rare illness, paroxysmal nocturnal hemoglobinuria (PNH), through a bulk-buy deal reached by the provincial premiers in 2011.