They do not check for "pot in your pee". LOL!
There are three things the medical is used for:
1. That the applicant will not cause excessive demand to the Canadian health care system (having serious, chronic and/or terminal illness requiring ongoing and expensive medical care) They assess it on the basis of it costing approximately more than $5000 per year to treat. FC spousal applications are exempt from this.
2. That the applicant does not have an illness that will cause a danger to the Canadian Public Health (SARS, other contagious disease that is active and easily transmittable). For ex. they check for TB and if they find a person has been exposed, do further testing to ensure that it is not active and the applicant has received adequate treatment (to also HELP the applicant).
3. That the applicant does not have an illness that will cause a danger to the Canadian Public Safety (severe and perhaps violent psychosis - schizophrenia with a history of violent episodes comes to mind).
Keep in mind that as doctors, their job is to also protect your health and advise you of any conditions you may have as well as recommend treatment for YOUR benefit, not just a pass/fail for Canadian immigration.