If my doctor is affiliated what can I do?

I have a suspicion that my doctor is prescribing me pills from a specific brand and it makes me suspicious that he's affiliated with this brand. How can find out if my suspicion is true, is it bad for me? What potential problems I can have and is it ethical?
So why do I think like that, first, this brand is persistent in his office, like a mouse pad, some installations, the calendar branded with same brand. Also, he handed me a box with 4 pills and told me I have to take them for a month which means I have to buy 26 more pills and I'm not sure I need those and he was pretty persistent repeating this 3 times. Actually the way he said it made me suspicious I guess.