Can I take phentermine (for weight loss) & Ritalin (for focus at work) together? I checked the Interaction Checker and there were none reported but I took them together today and I have sort of a headache. It could be from not eating or maybe I'm just thirsty but I just want to make sure I'm not doing myself harm. Not in a position to ask my doctor since my health insurance lapsed but I'm healthy apart from needing to lose 40 lbs. and I don't have any allergies/allergic reactions. I'm a careful person by nature so taking both of these things makes me wonder.