The procedure for the psychological evaluation of drug users is complex, because it involves obtaining the information necessary to develop an individualized intervention plan to the client's needs. But, drug users, and even more so those with legal problems, frequently offer distorted answers on the following dimensions: 1) The frequency and type of substances used; 2) The immediate psychological and behavioral consequences of consumption; 3) Deterioration of functioning due to drug use; 4) Quantity consumed; 5) Motivation for changing addictive behavior, but not motivation for treatment. Biased responses become a major issue when the scope of the study involves sensitive social issues, such as politics, religion and the environment, or personal issues such as drug use, deception and smoking. The tendency to lie of the respondents has a domino effect, in that it affects, firstly, the validity of the questionnaires and tests, and secondly it influences the conclusions and recommendations for possible psychological interventions and implicitly their effectiveness. Although, most people try to present themselves in a positive light (faking good), there are also people who present themselves worse than they really are (faking bad). Regarding the persons in the first category, the specialized literature mentions that they lie in order to avoid shame, guilt and social stigmatization, or due to the fear of not receiving a custodial sentence or a big pecuniary penalty. Thus, in the case of this category, the way it presents itself in society is influenced by two components: impression management and/or self-deception. Regarding the latter category, namely those who present themselves negatively, we often find those with a very pronounced self-critical sense, who come from a rigid, restrictive family environment, with very high standards and who often experience anxiety or depression. Therefore, it is desirable to apply at the evaluation meeting an appropriate tool to assess the level of social desirability and to separate the different aspects of this construct and to take into account personality variables in order to avoid interpreting social desirability only as a measure of counterfeiting.