The time inconsistency of decisions (TID) is the phenomenon, studied in various domains of economics, when a decision is optimal from the perspective of one moment in time and ceases being so in a subsequent moment. OBJECTIVES: The aim of the study was to check for the feasibility of prevailing of the TID in the pharmacoeconomic sequential decision problems as well as to identify the impact of this phenomenon on the process of decision implementation and its outcomes. METHODS: A formal model of sequential decision problems, both with and without uncertainty, based on a graph theory, was provided. In such a framework the decision problem is represented by a graph and a set of functions over the vertices representing the costs, effects and the probability distributions; decision alternatives are subgraphs; alternatives are described by the expected values of costs and effects; the rule of choice is to minimize criterion function of the expected cost and effectiveness, representing the preferences of a decision maker. The flow of time can be modelled by analyzing subsequent decision problems, called reduced problems, being the subgraphs of the original problem obtained by cutting the original graph in a certain vertex. RESULTS: There exist criteria susceptible to TID phenomenon, in particular the cost-effectiveness criterion is susceptible to TID in problems both with and without uncertainty and cost-benefit criterion or incremental cost-effectiveness criterion are resistant to TID in these kinds of problems. TID can lead actual decision makers to behave differently than advised accordingly to the model solutions and can make them choose actions that lead to pareto-nonoptimal decisions. CONCLUSIONS: The TID is immanently present in pharmacoeconomic decision problems as the widely used cost-effectiveness criterion is susceptible to it. It causes ambiguities in decision problem solving as the actual decision maker(s) may not stick to the model solution in a real life. The effect of this phenomenon can be pareto-nonoptimal behaviour.