Focusing on French toxicology, this paper analyzes the tensions, trade zones and synergies between chemistry and medicine during the first half of nineteenth-century. First, I analyze the contested status of medical chemistry in early nineteenth-century France, through two major reviews by Adrien de Lens (1786-1846) and Godefroy Coutanceau (1775-1831) on the application of chemistry to the different branches of medicine. I argue that it would be misleading and simplistic to characterize these authors as “vitalist” for their arguments on the limitation of chemical and physical laws in the explanation of life phenomena. In fact, as many early nineteenth-century physicians, Lens and Coutanceau questioned some recent medical applications of chemistry (particularly when dealing with physiology and pathology) while largely praising the advantages of chemistry in anatomy, hygiene, pharmacy and legal medicine. Focusing on the latter, I deal with the development of chemical analysis in toxicology and I show how chemical tests gained momentum in courtrooms and turned out to be perceived as the most reliable source of evidence in poisoning trials. During the first half of nineteenth-century, this form of proof successfully competed with two other sources of forensic evidence: clinical data and post-mortem examinations. The advances of chemistry in legal medicine were fraught with controversy, which was fueled by the different backgrounds and laboratory resources of nineteenth-century forensic experts, rather than by contrasting ideas about vital and chemical forces. The new high-sensitivity chemical tests required skilled hands and a high degree of competence which could only be gained by continued practice in well-furnished laboratories (like those at the Paris Faculty of Medicine). This was usually beyond the scope of the local physicians and pharmacists who participated as expert witness in poisoning trials. However, local experts could make the most of their direct access to the clinical history of the victims and, sometimes, their advantageous position in post-mortem autopsies. I argue that these features shaped the controversy between Parisian and local experts concerning the different sources of toxicological evidence. Following some of these controversies, the paper also analyzes the abundant and multidirectional flow of information, objects and practices between medical academies, chemical laboratories and courtrooms.