It interfenes strongly with the DMT, but it pushes the presence of the DMT effects away.
You're way much less
off with THC in your system (read: I'm talking about SMOKED cannabis), and it is more of a 'normal' psychedelic effect. With closed eyes, there are strong CEV's, but they don't pull you into it through another world. Nevertheless the effects may be very strong, like tripping balls on psilo or acid, but not a teleport to the DMT realm.
Tobacco however, increases the rush very powerful and the shut down of all you is suddenly there.
Usually your sweeting palms, increased breath and tense is a sign that something grand is going to happen. While with tobacco, there's more relaxtion so you have less determination signs of what's coming.
It is the moment in where NEO's body becomes glass, which raises intense fear for a few seconds. The DMT puts you through before you can even come up with a clue of what's happening. From the very first second of your dissolution, all is well.
I'd wait till you have an empty system to be sure, wasting DMT is not a favor.