I take that valium!
1 Valium just to allay some of the anxiety of the injection/ procedure should not interfere at all with your BP meds. And it should not lower your BP too much at all.
I have had a lot
of root canal and never had to go off my BP meds for them, and yes, I took a valium before I went in.
Unless you have other medical issues that would contraindicate valium, I don't think it should be a problem.
If you go with gas again, then you should ask your doctor if it'd be okay, just because the gas plus valium would be more sedating.
Why did you have to go off your BP meds on your last dental visit??