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Dark lips could be either genetic or secondary to a cause .If it is due to any allergy(cosmetic induced) then you should give a try with topical creams like steroids
or tacrolimus creams. Also change of tooth pastes helps at times. Do use a sunscreen in the morning.
If you don't respond to the above treatment then you can think of lasers as a last resort.The best lasers to go for would be Q-switched Nd:yag laser (1064nm) which is specific for pigmentation.I've myself treated patients with this with variable results.
So consult a dermatologist
who has specific kind of lasers.