There have been a few studies about the long term effects of heavy LSD use. Heavy LSD generally being defined as some total life time number of trips, usually a few hundred. The available results show that LSD does cause some changes in your brain, but they are give and take. Heavy users have brain wave profiles more consistent with a relaxed and anxiety free state. They have quicker visual response reaction times. They have a near complete lack of certain tactile perceptual illusions that are very common in most people. They also can differentiate between slight color variations better than average. There are several other positive long term effects as well, however I cannot recall all of them. They also are shown to have a somewhat decreased cognitive capacity for organizational tasks versus control groups. There are a few other negatives as well. In my personal experience, as a heavy LSD user, I would say that I have not really noticed any negative effects from LSD use, despite having tripped several hundred times over several years. As far as negative physical effects, they are essentially non-existent. There is of course the risk of HPPD, but it isn't horrible in most cases anyway and almost always fades away after you stop using. Of course there is also the risk of triggering a latent psychological disorder such as schizophrenia, however LSD will just lower the age of onset it wont cause it to happen if you were not already predisposed to it. Pretty much LSD is one of the safest drugs to take and with the least negative long term effects of essentially all other recreational drugs. Some may be about on par with it, but I cannot think of a safer recreational drug than LSD. The noted cognitive changes are give and take between positive and negative, and in most all cases are mild, and only in the heaviest of users (several hundred total trips in a lifetime).