Yes, the doctor can demand you work, even when under muscle relaxant side effects, unless you use paid time off (PTO) you earned, like sick days, to stay out of work, or your employer is covered by, you are eligible for, your condition justifies, and you use unpaid FMLA leave to stay out of work. Otherwise, except as above, the law does not guaranty you the right to miss work or not work, even for medical issues, and you could be disciplined, up to and including being terminated, for not working when your employer tells you to.