Contrary to popular belief, petroleum jelly does not cause eyelashes to grow any thicker, longer or faster. It adds a glossy sheen, coats the lashes and clumps them together so that light reflecting from them gives them a thicker and fuller appearance.
Petroleum jelly may also provide some hydration to dry lashes that can make lashes look livelier. However, prolonged use can actually cause lashes to dry if they are not exposed to air and water periodically.
Petroleum jelly can also be harmful if it gets into the eye. It can cause infection and blurred vision that may last for several days and can even cause blindness in severe cases. As an alternative to petroleum jelly, doctors recommend applying olive oil with a clean mascara brush to achieve the same effect, or using a medication such as Latrisse, which actually does help eyelashes grow.