First, I am not a doctor, and am only providing my opinion, based on my own experience with effexor.
I would not make another reduction any time soon. Your doctor has reduced your effexor by 50%. It is recommended by the experts in the field (Horowitz and Sorensen) to reduce by 10% or less, every 4-6 weeks, based on your withdrawal symptoms. "Replacing" it with Prozac is like substituting an apple for a banana - they are both fruits, but don't have the same components or work in the exact same way.
While this plan may work for some, it obviously is not working for you. I have been in your position, and know how terrible and scared you must feel. Please research the above people and try to find a dr/psychiatrist that knows about hyperbolic tapering. I feel for you.

Dear Casstate
I have gone thru this Effexor / Venlafaxine issue / experience and have, as a result, consulted Neuro-pharmacolgists World Wide over many years. Also, consulting this Site and another USA leading one with many years of Patient contributions. Base line appears that everyone is different in their make up & therefore reactions to this Drug. I have read about people on staggeringly high doses , then titrating off quite abruptly, with little side effects. How long were you on 150 mg? I am an extreme example - it took me well over 2 yrs to titrate off 75 mg Effexor , with no other meds to balance . I still get some legacy side effects - 2 yrs after stopping. Titration was overseen by my Dr and Pharmacist. From the symptoms you describe and your stated , very short cutting down of doses, I would think , based on feedback I have read , you are titrating off too fast and your body cannot adjust. Hence the adverse effects. It is also evidenced that the longer you are on effexor , the slower you need to take titrating off with the drop down dose. But as I say , I have never taken Prozac.

It seems to be different from one person to the next. I went from 75 to zero in one jump and had an absolutely miserable four weeks, and lingering reactions for a few following weeks. It's a long story, but in my case the drug had flipped me upside down and made me suicidal. A rare but known side effect. I had to get off of it in a hurry. I was able to plow through the vertigo and nausea and brain zaps because I knew my mood was improving, so I had that driving me.

On a side note, having seen no end of posts from people struggling to get off of that drug and not uncommonly failing to do so, I seriously question whether it should be FDA approved. No pharmaceutical intended to help someone should become a nightmare to quit using if it doesn't work. The drug itself becomes worse than the disease.

At bare minimum, doctors should be required to carefully explain to patients that if it doesn't work, withdrawal is potentially miserable and extended. I was never told this, and if I had been, would have probably thought twice about taking it. I was in a vulnerable place and my general provider just said "Here, take this. If you're not better in a few weeks get back to me." That was inadequate to put it mildly.

I'd like to see a national movement of those of us who have been on this medication and have had to quit and have struggled with it to get the word out. I'm not opposed to antidepressants, they do save lives, but if they have a potential to create harmful problems like Effexor does, they need to be handled with much greater caution.

@casstate I am so sorry to hear that you were going through such a difficult time getting off the Effexor. I went through a very similar situation about 15 years ago with the same drug. I was a mess. I cried all day at work. I felt as though my head was gonna explode. I couldn’t control myself at all and I’m so glad I got off of it. If you are struggling this much with the drug consult your doctor or psychiatrist whoever recommends you taping off and see if you can go in a slower rate because in my uneducated opinion, I think you’re going off it too quickly and you can always spread it out further over six or eight weeks but when they put you on this medication, they certainly don’t tell you about how difficult it is to get off of it because they don’t know they don’t have that issue. I wish you the best of luck. I will be thinking of you and praying for you. Please keep us posted as to how things go.