Fluoxetine is a selective serotonin reuptake inhibitor, that was the first such agent to be approved in the United States. It is also the agent with the longest half-life, along with its active metabolite norfluoxetine. In fact, a preparation called Prozac Weekly is available in the U.S., as it can be taken once a week. Consequently it is one of only two antidepressants (the other being agomelatine) that can be discontinued without any withdrawal syndrome being seen. It is also a potent inhibitor of CYP2D6, hence can interact with a wide variety of different agents including codeine, oxycodone, tamoxifen, aripiprazole and risperidone. It is considered the most stimulating (that is, most prone to wake people up, causing anxiety, insomnia, etc.) SSRI. Likewise it also inhibits CYP2C19 and CYP3A4; induces CYP2C9.