Drugs with more balanced selectivity for dopamine/norepinephrine and serotonin transmission, such as 3,4-methylenedioxy-methamphetamine (MDMA), tend to be less addictive, and they have mixed effects on ICSS responding depending on dose and stimulation frequency. Serotonin selective drugs, however, tend to result in either a lack of ICSS potentiation or depression of ICSS responding (a right-shift in frequency-rate curve), and these drugs are generally considered to be less addictive. One such example is fenfluramine, which was previously marketed as an appetite suppressant. Dopamine antagonists generally result in the depression of ICSS responding and a rightward-shift in the frequency-rate curve. This suggests decreased BSR and possibly increased aversive properties of the stimulation. Following chronic treatment with a dopamine antagonist, there is withdrawal-induced facilitation of ICSS, the opposite effect of what is observed following chronic treatment with stimulants.