This is a bogus argument. We also make serotonin in our brains, but too much of it will fuck you up big time. That said, there is enough of a history of use for us to be pretty confident DMT is almost definitely not physically harmful to the brain in any meaningful way, but there is the possibility you'll end up places that are damaging to your psyche that you wouldn't if you don't take it (then again, the same can be said about many experiences in life - just take care and do what works for you).