Sodium thiosulfate is a versatile chemical compound used in emergency medicine, cancer treatment, kidney disease management, water treatment, photography, and skin care. Its core chemistry centers on its ability to bind with and neutralize other substances, whether that’s cyanide in the bloodstream, chlorine in tap water, or unexposed silver in a photograph. Here’s how it works across each of its major applications.
Emergency Cyanide Antidote
The most critical medical use of sodium thiosulfate is treating cyanide poisoning. Cyanide shuts down cells’ ability to use oxygen, forcing the body into a crisis that produces dangerous acid buildup in the blood. Sodium thiosulfate works by donating sulfur to an enzyme in the liver, which converts cyanide into thiocyanate, a far less toxic compound the kidneys can then flush out.
In emergency settings, it’s given intravenously alongside sodium nitrite, which is administered first. The two drugs work as a team: sodium nitrite pulls cyanide away from cells, and sodium thiosulfate helps the body eliminate it. If symptoms return after the first round, a second dose at half strength can be given. Speed matters here. Treatment needs to begin as soon as cyanide poisoning is confirmed.
Protecting Children’s Hearing During Cancer Treatment
Cisplatin is a powerful chemotherapy drug, but it carries a well-known risk of permanent hearing loss, especially in children. Sodium thiosulfate can cut that risk nearly in half. In a clinical trial of children with a type of liver cancer called hepatoblastoma, 63% of those who received cisplatin alone developed some degree of hearing loss, compared to 33% of those who also received sodium thiosulfate. That’s a 48% reduction in risk.
The timing is precise and intentional. Sodium thiosulfate is given intravenously six hours after cisplatin treatment ends. This delay allows the chemotherapy enough time to attack the tumor before the thiosulfate arrives to neutralize the leftover cisplatin circulating in the body. In the trial, this delay preserved the cancer-fighting effectiveness of the treatment: overall survival and event-free survival were not compromised.
Dissolving Calcium Deposits in Kidney Disease
Calciphylaxis is a painful and serious condition that primarily affects people with end-stage kidney disease. Calcium builds up in small blood vessels under the skin, blocking blood flow and causing tissue death. The resulting wounds are extremely difficult to treat.
Sodium thiosulfate helps by binding to calcium and forming calcium thiosulfate, a compound that dissolves more easily than other calcium salts and can be cleared from the body. For dialysis patients, a typical regimen involves 25 grams given intravenously three times per week, infused during the last hour of or immediately after hemodialysis. This is generally a long-term treatment that continues alongside wound care and other supportive measures.
Removing Chlorine From Drinking and Aquarium Water
Outside of medicine, one of the most common uses for sodium thiosulfate is neutralizing chlorine in water. Municipal water systems add chlorine (and often chloramine) to kill bacteria, but that residual chlorine is toxic to fish, invertebrates, and beneficial bacteria in aquariums and ponds. Sodium thiosulfate reacts with chlorine to produce harmless byproducts, making the water safe.
Most aquarium water conditioners list sodium thiosulfate as their active ingredient. You can also make your own by dissolving about 32 grams of sodium thiosulfate in one cup of water. At standard chlorine levels, one teaspoon of this solution treats roughly 50 gallons, which works out to about two drops per gallon. Since municipalities sometimes spike chlorine levels during “chlorine pulses” (up to 4 parts per million), many aquarists use five times that dose as a safety margin: ten drops per gallon.
The same chemistry applies in larger-scale water treatment. Swimming pools, industrial cooling systems, and wastewater facilities all use sodium thiosulfate to dechlorinate water before it’s discharged into natural waterways.
Fixing Photographic Images
Sodium thiosulfate has been a cornerstone of film photography since the 1800s, where it’s known simply as “hypo” or “fixer.” When photographic film or paper is exposed to light, some of the silver halide crystals in the emulsion react to form the image. The unexposed crystals remain, and if left in place, they would eventually darken and ruin the photograph.
Sodium thiosulfate solves this by forming a complex with the unreacted silver halide, creating a large molecule that dissolves and washes out of the emulsion. Enough thiosulfate needs to be present in the solution to form what chemists call the “higher complexes,” because the simpler, smaller silver-thiosulfate compounds don’t dissolve well. Once fixing is complete and the print is washed, the image is chemically stable and can last for decades.
Treating Tinea Versicolor
Tinea versicolor is a common skin condition caused by an overgrowth of a yeast that naturally lives on the skin. It produces discolored patches, typically on the chest, back, and shoulders. Sodium thiosulfate applied topically at a 30% concentration has been used as a treatment for this condition, applied directly to the affected skin. It’s one of the older and more affordable options, though newer antifungal creams and shampoos have become more widely prescribed.
How the Body Processes It
When given intravenously, sodium thiosulfate distributes through the bloodstream quickly, with a distribution phase that lasts about 23 minutes. The liver partially breaks it down into sulfite and then sulfate, but most of the compound is eliminated by the kidneys through filtration. In healthy people, the elimination half-life ranges from roughly 16 to 80 minutes, meaning it clears the body relatively fast.
Side Effects
Sodium thiosulfate is generally well tolerated. The most commonly reported side effects from intravenous use are low blood pressure, headache, and disorientation. Some people also experience nausea, vomiting, a salty taste in the mouth, or a warm sensation spreading through the body during infusion. Prolonged bleeding time has been noted in some cases. Blood pressure is monitored during administration, and the infusion rate is slowed if pressure drops significantly.