The purpose of toxicovigilance is to monitor the acute or chronic toxic effects on humans of a mixture or substance—whether natural or synthetic, available on the market or present in the environment. Under current regulations, Article L. 1413-4 of the Public Health Code stipulates that the French Institute for Public Health Surveillance (InVS) organizes toxicovigilance in collaboration with the network of professionals listed in Section L. 1341-1. The system thus relies on information collected by the Poison Control and Toxicovigilance Centers (CAPTV), ensuring a 24-hour response to toxicological emergencies. This network also relies on the exchange of information and alerts with the Directorate General for Health, the Regional Health Agencies, the Health Safety Agencies, the Directorate General for Competition, Consumer Affairs, and Fraud Control (DGCCRF), the InVS’s international surveillance unit, and foreign toxicovigilance networks. A weak signal in pharmacovigilance may correspond to a health event, initially of limited scope, associated with exposure to one or more natural or synthetic substances, or even, in the absence of health effects, to emerging exposure to a biological or chemical agent available on the market or present in the environment, which may reveal a larger-scale health threat. Various situations involving the emergence of weak signals have been identified and will be illustrated with examples: (1) observation of one or more cases suggesting a causal relationship between the health event and exposure; (2) reports of cases, whether clustered or scattered, presenting the same clinical and biological picture (symptoms, syndrome, disease) for which a toxicological hypothesis is identified; (3) an emerging toxicological risk situation identified during a specific study. To assess the causality of the link, it is necessary to establish a precise account of the facts: contextual elements, chronology of events, symptoms, rigorous identification of products or substances, and metrological and analytical data confirming the hypothesis and the level of exposure. The identification and assessment of this type of signal require formal coordination among the various stakeholders. Channels for signal reporting by CAPTVs and for information exchange between the various institutions have been established. Criteria for signal validation, health threat assessment, and triggering an alert have been proposed. The mobilization of multidisciplinary and multi-source expertise is essential for the evaluation of weak signals in toxicovigilance. (R.A.)