NanoDropper makes one thing, and it is not a drop. The NanoDropper adaptor is a small cap that screws onto a standard eye drop bottle in place of the original tip, and releases a drop roughly sixty percent smaller than the bottle would on its own.
A standard eye drop is larger than the surface of the eye can hold, so a good share of every drop runs onto the cheek or drains away. A smaller drop means less overflow and more doses out of the same bottle, which matters most to anyone using a prescription drop on a set schedule, where the bottle is a recurring cost. Some people simply find it tidier, with less liquid on the lid and lashes.
The adaptor is sold in a pack covering two sizes, so it fits the two most common bottle neck diameters. It is reusable, and it moves across to the next bottle when you finish one.
Check the bottle you actually use before you buy. The adaptor is designed for standard round dropper bottles. Single use vials, gel and ointment tubes and some specialty packaging will not accept it, so it is worth looking at what is in your bathroom cabinet first.
If the bottle holds a prescription medication, mention the adaptor to the prescribing doctor or the pharmacist before you fit it. They are the right people to confirm whether it is appropriate for that particular product, and they can note it on your record so nobody is working from a different picture than you are.