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The Norwegian Directorate of Health is considering a national overdose warning system (OWS), to quickly alert users about substances with high overdose risks. Such a system would combine early detection of increased overdose risk and rapid notification to those at-risk. We conducted a systematic scoping review of research on existing OWSes. Aim 1 was to map current quantitative research on the effect of an OWS (number of users warned, number of overdoses, and notification speed). Aim 2 was to map qualitative research on users’ experiences. We used several machine learning functions to identify and assess 4102 references retrieved. Eleven studies met our inclusion criteria. The effect of OWSs on the number of overdoses was not possible to distinguish from the effect of existing overdose prevention measures. OWSs notified harm reduction groups or peer recovery support specialists as early as four hours after an increased risk was discovered. The number of users notified was largely unreported, most likely because data on this was not collected. From seven qualitative studies, patterns were identified across the main findings, related to preferences for language use and content, where notifications should come from, and the principles behind them. The evidence base, particularly relating to the effect of OWSs, is scarce. Without a sufficient knowledge base on effectiveness, we must be careful about making conclusions about which types of OWSs that should and should not be introduced in Norway. It can be viewed as a positive that the OWSs described in the included studies were heterogeneous. Creating an OWS does not necessarily have to involve the establishment of a new infrastructure, but rather links together well-developed, existing systems for monitoring and for harm reduction.