I am a sporadic amphetamine sulfate user to support my lows and depressive states.
In the end, all psychoactive substances are nothing more than tools. But if we find ourselves in the need to use them on a daily basis, I think they become something similar to a crutch. As far as productivity goes, if it's something that's productive for you, it might be worth it. If it's to be productive for others, no, don't. I can also say that amphetamine makes you "believe" that you are more productive and that makes it more difficult to stop taking it later.
But it's a half-truth, it makes you more productive because you focus more on the activity you do. But let's say, writing, you enter a creative maelstrom that has you writing for 10 hours. The thing is that later, when you get sober and read it, you realize that you have written entire pages that could be summarized in a single paragraph and would be more understandable.
By this I mean that it gives a false sense of productivity. Two hours sober and rest can be equal to or more productive than 10 hours on amphetamines. I would also like to say that with amphetamine sulfate I have also entered phases of amphetamine psychosis due to taking it for a long time. In my case, a very hard persecution mania, each person can be affected in different ways. The most dangerous are those that are least obvious, when we believe immutably in our own reasonings, reasonings created under the influence of substance.
Now let's say that I have created my rules to take it, always first thing in the morning so that it allows me to sleep and rest (important factor to enter psychosis is to have been sleeping for many days 2 or 3 hours + little food and drink), weigh the doses accurately and take it orally and not exceed more than 5 days in a row.
If you've been taking the retreat for a long time it sucks, but it's somewhat manageable, there's a lot of information available on what to expect and how to help you through the process, after the first week everything is easier.
As the other answers tell you, amphetamines and psychedelics are not a good combination. The most important part of a psychedelic experience is when we go back and give an interpretation to what we have experienced. If you're under the influence of an amphetamine, that interpretation will be much less flexible and highly distorted.
"Nosce te ipsum"