The material in phtochromic lens is silver halides and it is harmless to your eyes.
But it is not recommended to wear them too long. Your eyes may get tired as the pupil has to change as the lens color change.
The material in them isn't harmful at all. Responding to the other answers, neither is their function. They dim bright light, which makes your pupils dilate, but they also block all UV rays, so that isn't a problem. Furthermore, because they darken in the presence of bright outdoor light, but not in the presence of dimmer lights inside, your eyes barely have to adjust. That means your eyes are less tired, not more. Neither the material nor their function are harmful to your eyes in any way.