Another commenter pointed out the jist and carcinogenic nature of them. There's been many recalls in actually OTC and prescribed drugs due to them.
But it's mainly due to the "dimethlyamine" portion of DMT. It's a tertiary amine, but those can form nitrosamines in some conditions. It's mainly the synthesis of it that may make it more at risk. There's potential with a secondary amine in the presence of oxidizing agents. But I guess somewhat low looking at the potential synthetic routes.
Although with the amount people do and (probably the lack of frequency) it's probably fine.
Even better if it's extracted from a plant (but can still pose a nitrosamine risk depending on solvents used to extract the DMT).
My earlier deleted comment was due to me not clicking reply on your post... Just on the of one. Sorry.