Gastrointestinal (GI) side effects negatively impact dogs with lymphoma who are undergoing treatment with multi-drug chemotherapy. Up to 40% of dogs experience GI toxicity that requires dose reduction or treatment delays, which can increase the cost of care if hospitalization is required. As routinely prescribed medications inconsistently prevent GI toxicity, new options are needed. Disruption of the normal GI bacterial population, or dysbiosis, is implicated as a mechanism of GI toxicity. Dysbiosis induces GI structural changes, decreases intestinal absorption, and leads to increased permeability of the intestines. Microbiota metabolism is also disrupted, leading to abnormal fecal metabolic byproducts. These GI structural changes, hyperpermeability, and metabolism disruption increase the risk of diarrhea, GI bacterial translocation, and sepsis. Probiotics improve metabolome profiles, reduce severe diarrhea, and improve the GI barrier in humans undergoing chemotherapy but are an incompletely explored option in dogs. The investigators will evaluate the impact of a commercial probiotic on GI permeability and bacterial metabolism abnormalities in dogs with lymphoma being treated with multi-drug chemotherapy.