Abstract Metformin is the most common first-line oral therapeutic agent used in the treatment of type-2 diabetes, one of the most prevalent chronic diseases in North America. Post excretion, the compound enters wastewater treatment plants where it is partially bio-transformed by bacteria into guanylurea. Both metformin and guanylurea enter freshwater environments in wastewater effluent where they are available for uptake by aquatic biota. However, our understanding of the effects of metformin and guanylurea on aquatic life is limited. We tested the hypothesis that metformin and guanylurea can influence the development of zebrafish ( Danio rerio ), by assessing morphometrics, cardiac development, energetic state, and behaviour of early larvae. Embryos were exposed to environmentally relevant (0.4, 4, 40 μg·L −1 ) and supra-environmental (400 and 4000 μg·L −1 ) concentrations of metformin and guanylurea from the 4-cell stage (3 hours post fertilization; hpf), until first feed (120 hpf). Exposures to 40 μg·L −1 and higher of both metformin and guanylurea increased mortality. Metformin delayed hatching at the highest concentration tested (4000 μg·L −1 ). The incidence of spinal curvature increased with exposure to both chemicals at supra-environmental levels (400 and 4000 μg·L −1 for metformin; 400 μg·L −1 for guanylurea). Metformin and guanylurea exposure imposed slight bradycardia in early development, but did not alter oxygen consumption, ATP levels, carbohydrate levels, general swimming, light-dark movement, startle response, or thigmotaxis, irrespective of exposure concentration. The results suggest similar and low sensitivities of larval fish to both metformin and guanylurea. Apart from a small increase in mortality, these compounds impart a modest impact to the early-life stages of zebrafish that are largely limited to supra-environmental concentrations.