I haven't used such drugs during sessions, but have outside of them and done so a few times while being in therapy. I told my T each time beforehand and we talked about it after. I have also done them a few times before starting therapy.

I think the outcome of doing therapy on them depends on some factors, especially the relationship to the therapist, as well as whether the person taking the drug has done so before. I also think certain drugs would be better than others for this purpose (some take longer to wear off and I doubt most Ts would sit with you for 12 hours, for example).

These drugs usually have a lot of things going on besides the emotional component. So if you're doing them for the first time, that might be very distracting as well as uncomfortable. The visuals are one of the more fun things, but if you're doing therapy during that, it might be distracting or you might miss out on the visuals, which I'd personally think is a huge downside (it's something that you can only experience yourself, I've never seen images doing a good job of what it looks like and feels like, and it's a unique experience in my opinion). Also, there's a lot of physical effects, mostly an upset stomach, feeling restless, feeling like you constantly need to stretch, which might be distracting for therapy. If you're used to it, then an hour or so in, you'll be fine with just ignoring those effects, but if it's a first time thing, you might concentrate more on those things. This might also lead to a considerable amount of time used up just to manage those symptoms instead of doing therapy.

For all the psychedelics I've tried, I've also found that it's sometimes very hard to articulate things. Your thoughts jump all over the place, sometimes you just lose interest in whatever you were discussing. Sometimes you're very amazed by things that later on are not amazing at all (I remember one time laughing a whole lot about our neighbors having a boat in their yard, since it wasn't in the water, I thought that was very profound, but it isn't really, is it?). So a therapist that knows how to deal with that and has experience in leading such a conversation is certainly a must.

I think there are certain benefits that could be gained from doing therapy on these drugs. They can make you very emotional and also give you the 'clarity of mind' to see what you need to change (for example I always realize I should work out more, eat healthier, drink less) and the strength to actually do those things after. There is - at least for me - a pretty strong effect after the drugs have worn off, sometimes lasting over a month, where I'm more motivated to change things, work on myself and feel much less depressed. I could also imagine that if the therapist was trained in dealing with somebody who trips, there might be benefits from doing talk therapy. However, the relationship would have to be fairly strong for that to work properly, in my opinion. The client would have to feel comfortable enough to say whatever is on their mind without any fear of the therapist, since otherwise the client might become scared or feel unsafe with the therapist, which would probably not be beneficial. The T would have to provide for some things that normally aren't part of therapy as well, for example space to walk around in and rooms to switch to in case the client becomes uncomfortable, food and drinks. Since all of these things can become 'weird' under the influence of these drugs, the therapist needs to know about the possible reactions of their clients and plan accordingly. For example, cheese can look very unappetizing when moving and the client might not want to eat that, but fruits can be very fun to eat, which might in turn then also distract from the actual therapy. One might argue that if the session only goes for an hour, there's no need to provide food and so on, but in my experience, almost for the whole duration of the trip there is room for having important realizations, so only doing therapy for an hour would not be the best. Also, in the case it were done like that, the journey to and from the therapist would have to be dealt with somehow. Another thing to consider might be that boundaries tend to become kind of weird on these drugs, so it might be very confusing that one can't get a hug from their therapist or that he doesn't want to answer certain questions.

While I haven't really learned anything new about myself from talking to my T about the trips afterwards, I can say that therapy has helped me during them in managing my anxiety and caring for myself. And the trips themselves have shown me insights which I otherwise might never have had and have in general been an experience that I'd personally not want to miss. I have had the wish at least once to contact my T during a trip and share some things with him, but I was afterwards very glad I did not.

An additional thing to consider is that the drugs can have interactions with psychiatric medications, so that might be a thing to consider (I personally take an SSRI and have found to affect my trips by shortening them a lot).