This is a question commonly asked by patients especially the users of glaucoma eye drops since they are supposed to take the treatment for a long time.
How many drops a bottle will dispense depends on the viscosity of the drug and the size of the hole in the eye dropper. It is generally accepted that the drugs which have the viscosity similar to that of water will have about 20 drops in one milliliter which means we will have 100 drops in 5 mL.
To confirm and test, I took a few bottles of 5 mL eye drops and counted how many drops they provided individually. They ranged from 115 drops to 130 drops per 5 mL bottle. This does not mean that all eye drops of 5 mL capacity have 115 to 130 drops but we can safely assume that most of the 5 mL eye drop bottles have much more than 100 drops.
There are two important considerations here:
1. Many eye drops are available in volumes sufficient to last about one month. Depending on the dosage, they are available in 2.5 mL, 3 mL, 5 mL and 10 mL mostly.
2. Only one drop is enough to be instilled into the eye at a time and delivers the required dose. Any amount more than that will flow out of the eye because the conjunctival sac (the space between the eyelids and the eyeball) does not have room for more than one drop generally.
In the above context, let’s take the example of a few drugs. Timolol, for instance, is an anti-glaucoma drug used twice a day. For both eyes (glaucoma is mostly bilateral), it will be 4 drops a day and 120 (4 x 30) drops a month. So if you get 120 drops from the 5 mL bottle of Timolol, it should be sufficient for one month. Timolol eye drop is available mostly in 5 mL capacity.
Latanoprost/Xalatan eye drop is used once a day only. For both eyes, it will be 2 drops a day and 60 drops a month. It is mostly available in 2.5 mL capacity.
Ketotifen eye drop is used for allergy twice a day. For both eyes, 4 drops will be used in a day and 120 drops in a month. Ketotifen is available in 5 mL and 10 mL volumes (for one month and two months respectively).
The above analysis suggests that there are about 120 drops in a 5 mL bottle. This is fairly supported by my tiny experiment with some eye drop bottles. This obviously is not applicable to eye drops in suspension and gel forms.