How much does the pharma job pay and do they offer tuition benefits?
Given the plethora of drugs available for mental health, I don't think pharma experience will be irrelevant to your research interests. In any case, it isn't bad to get more work experience before starting grad school (and to save some money).
I was admitted to MPH programs straight out of undergrad, but deferred my acceptance and worked at a think tank for a bit before skipping the MPH to get a PhD. My work experience taught me really valuable skills (like statistical programming) that set me up for success later.