Always difficult when the question in the title is the opposite of the one at the end of the post. The answer is no, and yes. No one can stop you getting the medical attention you need. But as for most decisions in life, there are possible consequences. It's not unreasonable for a boss to ask to you reschedule if circumstances allow, but from what you tell us, that isn't possible. I'm suspecting that you value your life above your job so the decision is really already decided. Meanwhile the suggestions of union involvement, doctor's note, and saying you have to and will go, is all useful. I trust you have re-explained the necessity and circumstances to your boss after getting in touch with the hospital. It may be that they don't understand the urgency and inability to reschedule.