The aim of this paper is to present antidepressants and antipsychotics that can affect the prolongation of the QT interval, which can lead to sudden cardiac death, and to point out the importance of the role of pharmacists in counseling when applying this group of drugs. Relevant scientific and professional literature, as well as drug databases such as the Croatian Agency for Medicinal Products and Medical Devices (HALMED) and European Medicines Agency (EMA) have been systematically reviewed. First-generation antipsychotics have a higher tendency to prolong QT intervals compared with second-generation antipsychotics. Thioridazine, pimozide, sulftoprid and droperidol show a high risk, and haloperidol and chlorpromazine show risk to a lesser extent. Atypical antipsychotics that carry a high risk of QT prolongation are sertindole and ziprasidone. Risperidone has a slightly lower risk, and quetiapine, clozapine, and olanzapine have a negligible effect on the QT interval. Tricyclic antidepressants are more commonly associated with QT prolongation than other groups of antidepressants. Another group of antidepressants that can lead to QT prolongation are SSRIs. Citalopram prolongs the QT interval more than other SSRIs. Other risk factors that may lead to QT prolongation, such as female gender, older age, genetics, cardiovascular status of patients, and electrolyte imbalance, must be considered when prescribing these antipsychotics and antidepressants. Special caution is required when these antipsychotics and antidepressants are co-administered with drugs known to carry a risk of QT prolongation (e.g., group IA and III antiarrhythmics, antibiotics, antihistamines, etc.). Proper advice from a pharmacist could significantly reduce the risk of this side effect.