Praise be to Allah.
In a number of hadeeths it is narrated that the Prophet (peace and blessings of Allaah be upon him) used kohl and commanded others to do so. For example it is narrated in the Musannaf of Ibn Abi Shaybah, from the hadeeth of Anas, that the Prophet (peace and blessings of Allaah be upon him) used to apply kohl to his right eye three times, and to his left eye twice. This hadeeth was classed as saheeh by al-Albaani in al-Silsilah al-Saheehah no. 633. And it was proven in Sunan al-Nasaa’i (5113) and Sunan Abi Dawood (3837) and elsewhere, from the hadeeth of Ibn ‘Abbaas (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The best of your kohl is ithmid (antimony), for it makes the vision clear and makes the hair grow.”
Ibn al-Qayyim (may Allaah have mercy on him) said: Kohl protects the health of the eyes and gives strength and clarity of vision, and cleanses they eye of bad substances. In addition to that some types of kohl also serve as an adornment and if it is applied before going to bed this is even better. And ithmid is more efficacious than other types of kohl.
Zaad al-Ma’aad, 4/281.
It says in al-Mughni: It is mustahabb to use kohl an odd number of times. (al-Mughni, 1/106).
It says in al-Majmoo’: With regard to using kohl an odd number of times, there is a difference of scholarly opinion. It was said that it should be applied an odd number of times to one eye and an even number of times to the other, so that the total number will be odd. The correct view of the scholars who examined this issue is that it should be applied an odd number of times to each eye. Based on this, the Sunnah is to apply it three times to each eye. (al-Majmoo’, 1/334).
Shaykh Ibn ‘Uthyameen (may Allaah have mercy on him) was asked: Some eye doctors say that kohl is harmful to the eyes and they advise not using it. What do you say to them?
He replied: Ithmid (antimony) is known to be very good for the eyes. I do not know anything about other types of kohl. Trustworthy doctors are the ones whom we should consult on this matter.
And it was said of Zarqa’ al-Yamaamah, who could see as far as three days, that when she was killed, they saw that all the veins of her eyes bore traces of this ithmid.
(Majmoo’ al-Fataawa, vol. 17; Chapter on medical treatment and visiting the sick).
From the above it is known that kohl is good and is not harmful to the eyes, rather it is beneficial. But there may be some kinds of modern, manufactured kohl that contain some chemical ingredients which may cause physical damage and remove some of the benefits of kohl. Hence people nowadays should look for pure kohl and not regard every kind of kohl available nowadays as beneficial. And Allaah knows best.