Lactococcus casei (often referred to as Lactobacillus casei in older literature) is a probiotic bacterium that has been studied for its potential to support or treat Inflammatory Bowel Disorders (IBD), including Crohn's disease and ulcerative colitis. The rationale behind its use is based on its ability to modulate the gut microbiota, enhance intestinal barrier function, and influence immune responses that are often dysregulated in IBD.

Scientific validation exists but is limited and of moderate quality. Several animal studies suggest that L. casei strains can reduce intestinal inflammation, decrease pro-inflammatory cytokine production, and improve mucosal healing. Human studies are fewer and typically involve small sample sizes or are open-label in design. Some clinical trials have shown benefits, such as reduced disease activity and improved symptom scores in patients with ulcerative colitis, when L. casei is used as an adjunct to standard therapy. A notable example is the use of L. casei DN-114 001, which demonstrated some efficacy in maintaining remission in ulcerative colitis patients.

However, results across studies are inconsistent, and meta-analyses generally conclude that while there may be a positive effect, the evidence is not robust enough for strong clinical recommendations. The quality and quantity of data are insufficient to rate the evidence highly, but there is more support than for many other probiotics. Thus, L. casei is sometimes used, but as a complementary rather than primary therapy for IBD.