Hi Z,
No one knows for sure other than her injector and her hair dresser. Megan Fox is a very beautiful woman, but I would respectively disagree "that her lips look natural". They look like porn star lips. They detract from what should be the central focus of her face, her beautiful eyes.
Her upper lip is too big in relation to her lower lip (it should be about 70% the size of the lower lip), and her lower lip filling extends out too far laterally, approaching the appearance of a breakfast sausage.
You can achieve similar results with a number of fillers currently available. But remember that as with any tool, "It's the craftsman, not the tool" that will give you great results.
Try to keep your lips a "natural" size. Good luck and be well.
Dr. P
