Drug–drug interactions related to long-term therapies are of increasing concern. Psychotherapeutic drugs, licensed for the use in dogs for the management of separation anxiety and other behavioural disorders, are examples of drugs used in long-term therapies. In an in vitro system with canine P-glycoprotein (P-gp) expressing cell lines, three psychotherapeutic drugs with a different mode of action were tested for their ability to inhibit the canine multidrug transporter P-gp. At 10 μm, the selective serotonin reuptake inhibitor fluoxetine and the tricyclic antidepressant clomipramine inhibited P-gp for 41% and 59%, respectively. In contrast, selegeline did not inhibit the function of the canine P-gp.