Conflict situations do not only arise from misunderstandings, erroneous perceptions, partial knowledge, false beliefs, etc., but also from differences in `opinions' and in the different agents' value systems. It is not always possible, and maybe not even desirable, to `solve' this kind of conflict, as the sources are subjective. The communicating agents can, however, use knowledge of the opponent's preferences, to try and convince the partner of a point of view which they wish to promote. To deal with these situations requires an argumentative capacity, able to handle not only `demonstrative' arguments but also `dialectic' ones, which may not necessarily be based on rationality and valid premises. This paper presents a formalization of a theory of informal argumentation, focused on techniques to change attitudes of the interlocutor, in the domain of health promotion.