The main cause for dark lips is nicotine staining. Nicotine staining is caused by smoking and can only be minimized (and not eliminated) by using a cigarette holder. The only way to eliminate nicotine stains completely is by quitting. Excess caffeine, too, can stain your lips so if you are used to more than two cups a day, you will have to start cutting down on your coffee/tea intake. If the darkening has been sudden, it is very possible that it is an allergic reaction to a cosmetic product. Make sure that you only use lipsticks manufactured by reliable or reputed companies and always throw them out when they are passed their expiry date. Prolonged exposure to sunlight causes pigmentation which in turn results in darkening of the skin and lips. If you spend a lot of time outdoors, make sure that your lip balm has an SPF (sun protection factor) of at least 15.
In order to reduce the darkening you can apply a mixture of lemon juice and glycerin to your lips daily. Lemon juice is a natural bleach while glycerin helps to keep your lips moisturized. You can also exfoliate your lips with a toothbrush and some Vaseline. The skin on your lips is extremely delicate so make sure that you use a soft toothbrush (meant for toddlers) to exfoliate it gently. You can also make your own natural lips scrub using almond oil, honey and sugar. Honey contains antioxidants that protect your lips from damage caused by free radicals. Almond oil is an excellent moisturizer and sugar gives the mixture roughage and at the same time is easy on your skin.
Dryness and dehydration contribute to and accentuate darkening of the skin. Before going to bed every night make it a point to apply Vaseline petroleum jelly to your lips. Using a humidifier in your room can also be beneficial as it will keep your skin well hydrated. Most importantly make it a point to drink enough water and other fluids. Fruit juices and milk shakes will also help as they will not only provide you with the much needed fluid content, but they are also rich in vitamins and minerals that are essential for healthy skin. Your diet is in fact very important and unhealthy eating habits can aggravate the condition. Try to follow a balanced diet with a large amount of fresh fruits like grapefruit, oranges, apples, water melons, etc. and vegetables like to radish, turnips and lettuce.
answered by M W