A recent item on NZ TV program Seven Sharp highlighted potential side effect of using eyelash serums.
These over the counter products have been developed using prostglandins, a medicine used in glaucoma treatment. One of the side effects of using prostaglandin eye drops is eyelashes grow longer, stronger and darker. So serums have evolved to be applied to the lashes only.
However it good to read the fine print as there can be other side effects associated with long term prostaglandin use. These include darkening of skins and pigmentation around the eyes, and eye can become more deep set.
For more information see the following Consumer information.