In the medical literature "long term" is vague. Do you mean 3 months, 3 years, 30 years? The longer the time frame the more difficult to study. Then you need to define the frequency of use: light, moderate, heavy? Are we talking every week for 30 years? Then its further complicated by the fact that many pyschadelic users also frequently use multiple other drugs aka "polydrug users" in the literature.
Generally, studies on humans are notoriously difficult because we are not uniform and there are so many different factors that can skew results. In clinical trials, they attempt to get around this by having a huge number of study participants but this makes it very expensive. This is why big drug companies are mostly the ones doing them.
Since big drug companies are not really into DMT, most of "long term" studies on psychedelics are with a small number of participants. This means results are typically not statistically significant and should only be very cautiously translated to the general population, if at all.
That being said, it looks like there are some decent studies in the
Scientific Articles on DMT/Ayahuasca/Psychedelics thread already posted. And they already did all the hard work for us, not only searching for but giving access to all the studies, wow! I didn't see any large studies however here's an excerpted abstract from a review (looks at other studies):
Just my personal unscientific observations: humans have used psychedelics for a very, very long time. We don't even know how long since there's evidence it was before writing was invented! However, there are definitely some psychedelic users who are unstable and their usage worsens their condition. Some people could be helped by more sobriety in their life; constantly escaping from problems can make them worse.
Psychedelics, especially DMT, is strong medicine and should be respected. YMMV. Stay safe!