Hello there,
Darkening of lips can result from many reasons..
Smoking cigarettes
Excessive sun exposure,
Reaction from lipstick,
Too much caffeine etc..
If the darkening/Hyperpigmentation is limited to the lips then there is nothing to be worried about. But if this kind of darkening is visible in the skin of other parts of the body, it is a matter of concern.
Health Tips
Visit a physician if the symptoms persist in other parts of your body.