This article is the first of a series on pharmacological interactions involving medicaments commonly prescribed and/or used in odontology: vasoconstrictors in local anaesthetics and anti-inflammatory and anti-microbial analgesics. The necessity for the odontologist to be aware of adverse reactions as a result of the pharmacological interactions is due to the increase in medicament consumption by the general population. There is a demographic change with greater life expectancy and patients have increased chronic health problems and therefore have increased medicament intake. The presence of adrenaline (epinephrine) and other vasoconstrictors in local odontological anaesthetics is beneficial in relation to the duration and depth of anaesthesia and reduces bleeding and systemic toxicity of the local anaesthetic. However, it might produce pharmacological interactions between the injected vasoconstrictors and the local anaesthetic and adrenergic medicament administered exogenically which the odontologist should be aware of, especially because of the risk of consequent adverse reactions. Therefore the importance of conducting a detailed clinical history of the general state of health and include all medicaments, legal as well as illegal, taken by the patient.