Cancer cells increase nutrient consumption and metabolic fitness to support rapid growth and proliferation. Consequently, the tumor microenvironment (TME) accumulates metabolic by-products, such as lactate and ammonia, which are confined due to poor vascularization of the TME. In a recent study published in Science, the Haigis lab found that the metabolic by-product ammonia accumulates in the TME of breast cancer xenograft models and has functions far beyond a metabolic waste product. The authors used stable isotope metabolic tracing studies paired with LC-MS to detect that ammonia liberated in metabolic reactions is recycled and re-incorporated into amino acids. This recycling enables a cancer cell to maximize the biosynthetic potential of their nitrogen. Furthermore, the authors found that ammonia was not toxic to breast cancer cells and accelerated their rate of growth and proliferation in vitro and in vivo. This study re-orients the notion that ammonia is a toxic metabolic by-product, and highlights a novel, biosynthetic function for ammonia in cancer.