Why might a GP and a specialist give slightly different advice about combining medications?

GP's are generalists, they have a wider and good knowledge of most things. ADHD is a specalism within psychiatry with a real nuanced area of expertise. your GP might not know enough about your ADHD medication to make a specific reccomendation. I would always suggest to consult with a pharmacist, their specalist area of knowledge is medications and interactions. You can book time with them at most pharmacies and online consultations services exist too.