If a person is sick , how sick he/she has to be to miss fasting.
There is no fixed definition as to how sick a person has to be in order leave the Fast. It really depends upon an individual's condition. If the sickness is such that observing the Fast is too difficult then it would be permissible to leave the Fast. Also if the medical condition requires the use of medicine orally then it would also be permissible to leave the Fast.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America