Why does this eyelash-enhancer turn your eyes brown?

Put this drug on your eyelid, and it will help you grow longer eyelashes! Accidentally get some in your eye and it will turn your blue eyes brown. How does that happen?

I can tell that the internet knows I'm a woman because when I watch videos online, the accompanying commercials are always for skin cream and yogurt. Lately, though, I've noticed that they're also for a drug called Lumigan that will help me grow more eyelashes. This drug has been around for a few years, but got a shaky reception when people learned it could turn blue eyes brown. Although it gave the public pause, the side effect was well-known to glaucoma patients. The eyelash drug sprang from a type of glaucoma medication that was famous for turning eyes brown.

There are different types of glaucoma. Most cause damage to vision which is irreversible, but can be prevented. A common type is called "open-angle" glaucoma, in which increasing pressure inside the eye damages the optic nerve. Doctors prescribe prostaglandin analogs to relieve the pressure. The analogs mimic natural compounds which are found all over the body - but were found first in one particular place.

Ulf von Euler was the first person to isolate the compounds, and found them in human semen. Assuming that they were limited to semen, and had to come from the prostate, he called them prostaglandins. Actually, they're found in cells everywhere. They're messengers, but instead of carrying signal between cells, they signal within a cell.

Generally, prostaglandins help a cell do what it was designed to do in the first place. When a person gets a cut, they stimulate the production of platelets. When someone gets an infection, they stimulate cells to cause fever and signal pain. They stimulate the cells on the lining of the gut to increase the amount of mucus production. They increase blood flow in the kidneys. Whatever the cell does, they make it pick up the pace. In the case of glaucoma patients, they make the cells in the eye form a bucket brigade, relieving the extra pressure on the nerve by forcing the cell to get rid of its excess fluid.

Making More Melanin

But while they're in the eye, prostaglandins also come into contact with other cells. These cells are called melanocytes, and they produce melanin, the pigment that darkens the eyes. The prostaglandins often stimulate the melanocytes to produce more melanin, changing the color of the eye. This is especially common in people with some eye pigment already - so people with greenish or hazel eyes are more likely to have their eyes turn brown than the completely blue eyed.

Glaucoma patients noticed, while checking their eyes for signs of darkening, that the prostaglandin analogs stimulated the cell along their lids to grow hair more enthusiastically, making their eyelashes longer and more numerous.

A quick look at mascara sales showed that any product that can make more eyelashes was going to sell, so prostaglandin analogs became cosmetic. People taking them were urged never to put the drug on the eyeball, or apply it to the underside of the eye. The stimulation of melanocytes still works on the skin, which is why some people noticed darkening of the eyelids where the drug was applied. If someone misapplies the drug, or if they rub or blink it into their eye, it can stimulate the melanocytes there and darken their eyes to brown.

Via Elmhurst, Experimental Dermatology, The FASEB Journal, All About Vision.