Summary: In some ways, it’s hard to compare Zoloft vs Prozac. Both are fairly well tolerated but there are differences in their side effect profiles. Both can be effective and both take upwards of six weeks – sometimes more – to exert their full effect. Both are available as generic and are relatively inexpensive.
Dear Curtis: My doctor and I have decided to try me on an antidepressant. We’ve discussed Zoloft because it’s used by so many people. But I also wanted to know about Prozac. Are they any significant differences between these two?
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Zoloft and Prozac are actually both in the same class of antidepressants called Selective Serotonin Re-uptake Inhibitors.
What that means is that the hormone serotonin is usually taken back up by your body. In theory, if you can increase the levels of serotonin in your body it can help with depression. So both of these drugs slow down the rate at which your body ‘takes back’ it’s serotonin.
As far as the differences between the two I’ve found them both to be very popular with folks (I’d give the edge to Zoloft these days and especially it’s generic sertraline) but they can have different side effect profiles.
Comparing Side Effects
In general, I would consider Prozac (fluoxetine) to cause more anxiety. Zoloft (sertraline) causes more dizziness, headache and stomach problems – especially diarrhea. Oddly enough, Prozac also caused a high rate or rhinitis in studies.
Here’s a short table comparing the side effect profiles of Zoloft vs. Prozac. Please understand that these results are pooled from different studies and aren’t based on head-to-head trials between the two (1,2).
| Side Effect | Prozac | Zoloft |
| Dilated Blood Vessels | 2-3% | <0.1% |
| Anxiety | 12% | 4% |
| Dizziness | 2-10% | 6-17% |
| Fatigue | 0 | 10-16% |
| Headache | 13-24% | 25% |
| Insomnia | 9-24% | 12-28% |
| Nervousness | 3-14% | 5% |
| Diarrhea | 2-11% | 13-24% |
| Flatulence | 3% | N/A |
| Pharyngitis | 6-10% | N/A |
| Rhinitis | 16-23% | N/A |
| Weakness | 8-14% | >1% |
| Flu Syndrome | 3-12% | N/A |
Guessing at Effectiveness
Personally, there is no reliable way I have ever seen to determine which antidepressant will be more effective than the other. A lot of doctors and pharmacists have their own personal preferences – but as far as anything scientific I’ve found that patients often just have to start taking one medication. See how they do. And then reevaluate.
In general, you’ll find about a 60% to 70% success rate with either of these antidepressants. Again, this is a broad brush stroke statistic and is based on my experience. But I think most clinicians would probably throw out similar numbers.
Both drugs take a while to start working. Four weeks for a full effect would be considered early in my opinion. I’ve found that most folks get the best results by about week six. If it doesn’t appear to be working that well at that point then taking it longer probably isn’t going to do much good.
References:
- Prozac [package insert]. Indianapolis, IN: Eli Lilly; November 2003.
- Zoloft [package insert]. New York, NY: Roerig; September 2003.
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