Normally it's melanin, a brown pigment, that determines the color of the skin you're in, and it's also what causes you to tan (that's the skin's way of protecting itself from the sun's rays). But when you take tanning pills you completely change the way you develop a tan -- there are no ultraviolet rays or melanin involved. In fact, you may not even develop a brown tan; rather, you may end up looking more like an Oompa Loompa.
It's the canthaxanthin, the active ingredient in tanning pills, that simulates a tan -- just as it's used as a color additive to deeper the saturation of store-bought tomato sauce and added to a chicken's diet to intensify the color of its egg yolks, it can be used as a way to pigment your skin [source: WebMD]. Consume it in large amounts and it will give you that fake orange-brown, sun-kissed glow. The way it works is like this: The body dissolves canthaxanthin in the fat (the adipose tissue) just beneath the epidermis, and when we've consumed the pigment in large quantities it begins to accumulate in that fatty layer, tinting the color of your skin [source: Drugs].