According to the FDA, the Miracle Mineral Solution produces an industrial bleach:
The product, when used as directed, produces an industrial bleach that can cause serious harm to health. The product instructs consumers to mix the 28 percent sodium chlorite solution with an acid such as citrus juice. This mixture produces chlorine dioxide, a potent bleach used for stripping textiles and industrial water treatment. High oral doses of this bleach, such as those recommended in the labeling, can cause nausea, vomiting, diarrhea, and symptoms of severe dehydration.
Health Canada issued a similar warning:
According to the information provided on the company's website, Miracle Mineral Solution is a 28% solution of sodium chlorite. Health Canada advises that sodium chlorite is a chemical used mainly as a textile bleaching agent and disinfectant.
The U.S. Food and Drug Administration is warning consumers not to take Miracle Mineral Solution, an oral liquid also known as “Miracle Mineral Supplement” or “MMS.” The product, when used as directed, produces an industrial bleach that can cause serious harm to health.
The FDA has received several reports of health injuries from consumers using this product, including severe nausea, vomiting, and life-threatening low blood pressure from dehydration.
Consumers who have MMS should stop using it immediately and throw it away.
An update from Canada as of 29 November 2018:
Stanley Nowak has become the first person convicted under the Food and Drug Act for marketing, packaging and selling the so-called Miracle Mineral Solution, a controversial tonic that's been marketed as a cure for a variety of illnesses, diseases and conditions, including AIDS, cancer and autism.