L-cysteine, a semi-essential amino acid, has been studied for its potential to support the treatment of gastric ulcers, primarily due to its antioxidant and mucosal-protective properties. Scientific interest in L-cysteine for ulcer treatment is rooted in its role as a precursor to glutathione, a major intracellular antioxidant crucial for cellular defense against oxidative stress—a factor implicated in ulcer formation and delayed healing. Animal studies have shown that oral or intraperitoneal administration of L-cysteine can reduce the severity of experimentally induced gastric ulcers, likely by enhancing glutathione levels in the gastric mucosa, scavenging reactive oxygen species, and improving mucosal integrity.

Some in vitro studies and rodent models also suggest L-cysteine may increase mucin secretion and promote healing of the gastric lining. However, clinical evidence in humans is limited. There are a few early clinical trials and case reports that suggest some benefit of L-cysteine supplementation in ulcer patients, but these studies are small and often lack rigorous controls. Most mainstream ulcer treatments rely on acid-suppressing medications and antibiotics (for H. pylori), with L-cysteine not currently recommended as a standard therapy.

In summary, while there is a plausible biochemical rationale and some preclinical evidence for L-cysteine's ulcer-protective effects, high-quality human clinical trials are lacking. Therefore, the scientific validation is modest (rated 2/5), and more research is needed before L-cysteine can be recommended for ulcer management.