Bismuth (symbol Bi) is a chemical element classified as a heavy metal. Unlike heavy metals such as lead or mercury, elemental bismuth has very low toxicity. Bismuth has various commercial uses, but its most common application is in pharmaceuticals. The compound Bismuth Subsalicylate (BSS) is the active ingredient in common over-the-counter medications used to treat temporary digestive issues. The risk of systemic toxicity from typical therapeutic use of these compounds is extremely rare.
Forms of Bismuth and Their Safety Profiles
The potential for bismuth to cause harm depends heavily on its chemical structure, specifically whether it is in its elemental or compound form. Elemental bismuth is stable and has a good safety profile, often used as a non-toxic replacement for lead in industrial applications. In medicine, bismuth is administered as an insoluble compound, such as Bismuth Subsalicylate, not as the pure element.
When ingested, Bismuth Subsalicylate breaks down in the gastrointestinal tract, separating the salicylate component from the bismuth component. The salicylate part is readily absorbed into the bloodstream, where it exerts an anti-inflammatory effect.
The bismuth component forms highly insoluble salts in the digestive system. This low solubility is a critical chemical feature that dramatically limits how much of the heavy metal can be absorbed into the body. This mechanism is the primary reason why therapeutic doses of these medications are considered safe. The toxicity risk is therefore a function of the specific compound’s structure and the administered dose, not an inherent property of the element.
Signs of Acute and Chronic Bismuth Overexposure
Despite the element’s low toxicity, overexposure to bismuth compounds can lead to two distinct forms of systemic poisoning, depending on the dose and duration. Chronic, high-dose exposure, typically from long-term use of certain absorbable bismuth salts, primarily targets the central nervous system, leading to a condition known as Bismuth Encephalopathy. This neurotoxicity manifests with symptoms, including confusion, memory impairment, and a loss of coordination called ataxia, which results in difficulty with walking or standing.
Other neurological signs of chronic toxicity include involuntary muscle contractions (myoclonic jerks) and slurred speech (dysarthria). Acute toxicity, which follows a massive, single overdose, is more likely to cause significant kidney damage (nephrotoxicity). This acute kidney injury can present with reduced urine output (oliguria), accumulation of waste products in the blood (azotemia), and tubular damage called Fanconi’s syndrome.
These severe systemic effects must be distinguished from the common and harmless side effects of Bismuth Subsalicylate use. The bismuth portion reacts with trace amounts of sulfur in the mouth and intestines, forming bismuth sulfide. This black substance causes temporary, non-toxic discoloration of the stool and the tongue. Neurotoxicity and nephrotoxicity are linked to blood bismuth concentrations far exceeding those produced by recommended dosing.
How the Body Handles Bismuth
The body’s physiological processing of bismuth explains why toxicity is uncommon with therapeutic use. When Bismuth Subsalicylate is taken orally, the amount of the bismuth component absorbed into the bloodstream is very low. Studies show that typically less than one percent of the bismuth dose enters the systemic circulation.
The vast majority of ingested bismuth remains unabsorbed and is efficiently eliminated via the feces. The small absorbed portion enters the circulation, is not metabolized, and distributes to various tissues, with the highest concentration eventually found in the kidneys.
The absorbed bismuth is excreted primarily through the kidneys, but this elimination process is slow. The half-life of elimination can be as long as twenty days, meaning repeated dosing can lead to slow accumulation in the body. However, the body’s effective mechanism for limiting gastrointestinal absorption prevents this accumulation from reaching toxic levels, unless a person has significant pre-existing kidney impairment or consumes massive doses over an extended period.