Is it allowed for an employer to fire an employee if they leave mid shift due to medical reasons (with manager approval).

Your employer is not allowed to fire you for missing work due to medical reasons. If terminated, you would be owed severance and may be entitled to significant human rights damages. I would encourage you to visit the severance pay calculator our firm has developed to see what you are owed if terminated (www.SeverancePayCalculator.com). If you have been terminated, I would encourage you to call us right away and book a consultation with one of our employment lawyers.