Pre-placement Medicals are typically included as a condition of employment in job offers, where the employer makes the offer conditional on the employee being found 'medically fit' for the job. Thus, an employee declining the pre-placement exam is also declining to meet the condition and therefore, to accept the job.
An employer is entitled to make a job offer with conditions and require that the prospective employee meet those conditions in order to get (or in some cases keep) the job. For example, a company that is hiring someone to drive a vehicle (like a delivery truck) is legally allowed to require that the employee show proof of a valid driver's license at time of hiring, and maintain a valid driver's license throughout their employment. The same rules apply for a Preplacement medical.
An employee does have the right to refuse to participate in the medical - in which case the condition is not met, and the employer is not obligated to hire the employee. So yes, an employee can refuse a preplacement medical - but that also means that in doing so, they are turning down the job offer.
Note that in Ontario the rules for designated substance medicals (like asbestos, silica, etc.) are different and that an employee exposed to designated substances can decline to participate in the medical surveillance program, without adverse consequences.
Contact us to learn more about pre-placement or post-offer medicals.