The drive side pedal is right threaded or normal threads. Are you using a open end wrench or a hex key from the other side? With an open end wrench, you'd loosen it in the normal direction, counter clockwise (lefty loosey, righty tighty) for normal threads. If you are using a hex key and looking at the back side of the crank arm while you turn it, then you'd rotate it as if you were tightening it, or clockwise.
I don't like to use a hex key when high torque is involved. I've had too many setscrews and bolts with socket head caps that are fully counter sunk get a crack in them and when trying the loosen them. The crack opens and grabs the side of the hole and won't let the bolt move. Pedals seem to have more material between the socket and edge so maybe not as much of a problem with them., but if I have flats on the pedals, I'm using those with a open end wrench.
Edit:
Okay, think reverse of what I said since apparently I misread and thought it was the drive side that was your issue.