He's probably just WORN OUT from being on tour for so long. Opeth hasn't ever toured this much, and for a young guy like Lopez, it's gotta be a new and stressful outing. Personally I don't blame him for not wanting to play in Jordan, but I doubt that drugs had anything to do with it. I've had panic attacks in the past and they always occurred when I was dead sober.
Either way, best to Martin, he's an amazing drummer, and all the best to Opeth, my favorite damn band.