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Acute manifestations of various psychopathological symptoms require detailed differential diagnostic procedure, since their cause is found to be somatic in several cases. These adverse events during the treatment are often the side effects of the prescribed drugs or drug-drug interactions. In the presented case report, the patient had complex visual-auditory hallucinations two days after the initiation of tramadol-clarithromycin coadministration and these transient symptoms were repeated for two days. After the interruption of the administration of these drugs, the symptoms disappeared in two days, without the administration of any kind of psychotropics. These two drugs by themselves may cause hallucinations, and because both of them are metabolized by the same enzyme (CYP 3A4) in the liver, symptoms were worsened by the drug-drug interaction. The reason of this effect is that tramadol is the substrate and clarithromycin is the inhibitor of the CYP 3A4 enzyme. Medical examination results (physical examination, ECG, blood samples, CT scan, EEG) could not be causally related to the symptoms. Suspected risk factors were the old age of the patient, the condition of his brain and the interactions with other previously prescribed drugs. This case report calls the attention of clinicians to the fact that in vitro drug-drug interactions in vivo can produce clinical manifestations more often then taken into account.