2C-I can only get so far
Personally, I've found that 2C-I can only take me so far, in terms of depth of introspection, and that's regardless of the dose taken. In that sense it's a bit like pethedine (demorol); while pethedine is a good painkiller, no matter how large a dose is used, there are some pain that it's not strong enough for; that when you have to use morphine instead.
2C-I will not take you as far as 2C-E or mescaline, no matter how much you take.
Last year, I made up some sterile 2C-I soln., to use in combination with ketamine in order to better recall the experience (2C-B works for this purpose, and 2C-I's not bad). I eventually decided against using it, but to predose orally, a couple of hours beforehand.
So, one sat night, I decided the time was right for a ketamine/2C-I exp (other half was at work related conference all weekend). I took 5mg of 2C-I at about 5pm, and at 7pm drew up 1ml of sterile 50mg/ml ketamine soln into a diabetic syringe for IM administration. After inj into a buttock, I lay down and waited for the ketamine to envelope me. Something was wrong, then it hit me (thought, not drug); I'd put the 25mg/ml sterile 2C-I soln into an empty vial that used to contain ketamine solution, but forgotten to relabel it. After about 30 seconds of panic I realized that I'd just injected 25mg of 2C-I on top of the 5mg oral dose. I immediately swallowed 20mg of diazepam and phoned a friend, asking if he could come to mine, as I might need his help. If the 3X potency increase gained by snorting 2C-I also holds true for IM injection, then that night I had the equivalent to an 80mg oral dose.
Visually, the effects were so strong that at times everything in my field of vision was reduced to elemental shapes (squares, triangles etc) that were vivid neon hues, and I thought I was going to do the big spit a couple of times due to the body load, but I never got anywhere near the degree of introspection/chaos produced by 12.5mg of 2C-E (although what I did get lasted for what seemed an eternity - actually about 10 hours).
So for me at least, there is a definite limit to 2C-I's effects (and I label things as soon as the contents are added ever since).
No lasting HPPD, or any other effects, so (as well as being very lucky) 2C-I does not seem to be too acutely toxic either. Had that been 2C-E, it would have ended in tears (and I'd most probably still be crying!).
That said, I wouldn't recommend that anybody repeat what I did (I still can't believe my stupidity re: re-labelling the vial)