Prozac

fluoxetine

General Information

Prozac is the best known of the SSRI antidepressants. It was the first to be released in this country, and is still commonly prescribed. See the general page on SSRI's for more information pertaining to this class of antidepressants.

Unique Properties

Prozac has several properties which distinguish it from other SSRI's. Is is very activating, which makes it a good choice for people who have very low energy. However, may be more likely to increase nervousness, insomnia, and anxiety than some of the other medications in this class, especially at higher dosages. Prozac is the best studied of the SSRI antidepressants in pregnancy. While it has not been demonstrated to be entirely safe, its use during pregnancy may be appropriate under certain conditions.

Long Half-Life

Prozac exerts its effects longer than any of the other SSRI antidepressants. Prozac itself has a 2-3 day half-life in the system, and one of its active metabolites, norfluoxitine, has a 7 or 8 day half-life. What this means is that once a person starts taking Prozac, it takes about a month for it to build up the body, and once it is stopped, it takes a month or two to completely eliminate it. This has both advantages and disadvantages. The fact the medication builds up slowly may help to reduce early side effects (although it is still often best to start Prozac at a low dosage). Its tendency to be eliminated slowly means missing a day's dose has little effect, and that dosage can be fine-tuned, for instance, by taking it two out of 3 days. In addition, it may be less likely to cause the withrdawl symptoms we see with other SSRI's if they are stopped abruptly. The main disadvantage of this long half life is that, should the medication cause an undesireable side effect, such as excessive activation, it can take a long time to get it out of the system. The real clinical impact of this is debated, but most psychiatrists would feel less comfortable prescribing Prozac than the other SSRI's if there was a concern about the presence of bipolar illness or a mixed affective disorder.

Another advantage of Prozac's long half life is that it appears less likely to cause SSRI Discontinuation Syndrome. In fact, sometimes one or two doses of Prozac can be used to eliminate the side effects of stopping one of the other, shorter half-life SSRI antidepressants.

Specific Drug Interactions

As compared to the other SSRI's, Prozac is a particularly strong inhibitor of one of the liver enzyme systems which breaks down other drugs (the cytochrome P450 2D6 system). As a result, it may be more likely to interact with other drugs than some of the other SSRI's, and adjustments in dosages may be necessary. These include some tricyclic antidepressants, antipsychotics, certain cardiac antiarrhythmics, Dilantin, Tegretol, some beta blockers, codine, and trazadone. None of the SSRI's should not be combined with Seldane, Hisminal, and Propulsid.

Like all SSRI's, Prozac can cause the Serotonin Syndrome if mixed with other agents which increase serotonin.

• Prozac Monograph