Ritalin is fine. The provision forbids “unlawful drug use” at the federal level, so if you have a valid prescription to treat a medical issue, then it’s not unlawful. Despite being legal for use in PA, medical cannabis is still prohibited federally, thus being a cannabis user would therefore technically prohibit you from ownership of firearms. The only caveat I’m aware of is regarding psych meds (like Ritalin) is if you have been hospitalized for a mental health issue— that will also prohibit you from ownership.