I'm not sure exactly what the policies in Canada are. In the US, it is not an absolute barrier, but you have to show that you can fulfill your duties WITHOUT medication, and you have to have been off meds for a while (typically at least a year) before you sign up, and it becomes much harder to apply for certain elite units or positions.
If you believe you may have ADHD but do NOT want to be labeled, I recommend learning as much about the condition as possible. Perhaps you can find someone to help you learn non-drug management techniques, a professional organizer or some such. Some people are able to use caffeine in place of prescription stimulants.