Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Acid reflux does not break the fast. It is like vomiting which is not self-induced.
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Acid reflux does not break the fast. It is like vomiting which is not self-induced. Since you have no control over acid reflex, it does not break your fast.
However, if anything comes to your mouth, then you should spit it out.
Allah says, {Allah does not burden anyone what he or she has no strength to bear.} (Al-Baqarah 2:286)
Prophet Muhammad (peace be upon him) said, “Allah excuses my Ummah of actions done out of unwilful mistakes, omissions out of forgetfulness, and that which they do under compulsion, and thus have no control over them.” (Ibn Hibban)
Almighty Allah knows best.