From all posts I recall about this in the past it doesn't work well. I think it does still work, but it's unpredictable how much more you need. I couldn't tell you why since rectal bypasses DMTs enzymatic nemesis for the most part. Jamshyd posed a similar question about rectal ketamine years ago. Both ketamine and DMT work nasally, but poorly rectally (w/ketamine being about equipotent to oral). The best guess the people in that thread came up with is that it's simply poor absorption. Something about the chemical properties of ketamine (and perhaps DMT) makes it absorb poorly in that tissue as compared to the mucus membranes of the nose (this trend is somewhat reversed with many other compounds, with rectal often having significantly higher bioavailability than nasal [e.g. MXE, many opiates, and, I think, amphetamine]). It may be possible to vastly increase absorption by mixing DMT with something like PEG-g-chitosan (emphasis on "something" because I really have zero idea - just saying because I've heard it mentioned that it works for other drugs).