What is the definition of a sickness which allows a person to break their fast?

• A sickness that causes difficulty in keeping the fast, such as a fever that will not go away without medication. It is not merely a mild and inconvenient ailment.
• A sickness that would make it harmful to fast. For example, a person needs to take needed medication.
• A sickness that is not currently severe, but there is a potential for it to become difficult or harmful for the person to keep the fast. The person themselves determines if that potential exists. For example, a migraine sufferer knows from past experience that once the migraine starts developing its only going to keep on getting worse until the fast is broken.

Levels of sickness and permissibility of fasting:

• mild – such as minor headaches and body aches. It is impermissible to break the fast
• difficult – such as a fever. The majority of scholars say the person is not obligated to break the fast. Imam Ahmad says that the person is recommended to break the fast.
• harmful – it is impermissible not to break the fast because Allah (swt) said “….and do not kill yourselves (or one another). Indeed Allah to you is ever Merciful.” [an-Nisa, 4:29]