Via Reddit, someone going by the username of functionaljunkie offers a consumer report: Multi-Week Carfentanil Experience Report. Excerpt and then a comment:
There really seems to be little real useful information about carfentanil from a user's perspective aside from the usual paranoia about its potency. So here I am wanting to share some information about my experience, much of this is probably completely unknown to humans in general, particularly because this drug has not been tested in humans. I imagine much of my experience will be of interest to those in the recreational drug research business.
I want to say that I am not in any way endorsing the use of carfentanil. It can be a highly dangerous substance and could lead to a dependency that is very hard to break, not to mention death from overdose. Please, if you MUST experiment with this drug, take care, and get a LOT of Naloxone beforehand.
I have been an opioid addict for nearly 13 years now. I have tried many different opioids and have kicked the various substances many times as well, staying clean for several months at a time until the stresses of life brought me to the point of relapse again.
I first tried carfentanil near the beginning of the year, and was on it for about 3 weeks. Then I was unable to obtain any for a time and ended up taking a month break. Now I have been on it for almost 7 weeks. The attraction to carf is because price is great for the potency and the half-life is something like 7.7hrs, meaning it lasts a long time once you take it. If you are coming from a fentanyl or heroin habit it's almost too good to be true.
The experience taking it nasally was similar to fentanyl at the beginning, causes a bit of euphoria, drowsiness, and potentially a sort of amnesia effect where one doesn't remember what one did in a dream-like state. Just like fentanyl, carfentanil peaks fast and drops off quickly. The primary difference is that carfentanil gives a sort of warm, pleasant feeling for many hours afterwards, and you can go for 8-12hrs without needing to re-dose.
Much like with fentanyl, I liked the peak feeling. Ignoring the fact that I should not have to re-dose for many more hours, and noticing that all the strong sedative effects were gone, I began dosing nearly as frequently as with fentanyl, say every hour or two. At first one would think that this is extremely dangerous, but the respiratory depression all occurs within approximately the 1st hour with carfentanil, and tolerance develops quickly. After a while the high is not nearly as good and it mostly makes one drowsy, but the nice kind of drowsy, if you know what I mean.
It seems clear that a sophisticated international market exists for distributing and selling opioids, including fentanyl and carfentanil. We tend to see only the fatalities and serious overdoses, but they must be a small fraction of the total market.