All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Scholars provided two opinions on whether using eye drops invalidates fasting or not:
First: The Maliki and Hanbali scholars are of the opinion that using eye drops during daytime in Ramadan invalidates fasting if it reaches the throat. They argued that the eye is considered a gate to the stomach, even though it is not a regular one.
Second: According to the soundest opinion of the Hanafi scholars as well as the opinion of another group of the scholars using eye drops does not invalidate fasting, even if one finds its taste in the throat. They argue that such an action does not go against the objective of fasting. It is also the opinion adopted by Shaykh-ul-Islam Ibn Taymiyyah and a group of the contemporary scholars.
The preponderant opinion is that using eye drops does not invalidate fasting. However, it is better to delay it to the nighttime so as to stay away from the difference in scholarly opinions. It is also better to use a type of eye drops that does not reach the mouth and to spit it out once its taste is found there.
Allah Knows best.