Smoking causes oxidative stress, which hampers the blood supply that keeps skin tissues from looking supple. It also affects the production of collagen, which results is saggy looking skin ridden with wrinkles.
Cigarettes have carbon monoxide, which hampers oxygen supply in your skin while the nicotine also reduces blood flow. This causes your skin to turn dull and also deplete the effects of nutrients like Vitamin C, which is known to keep skin healthy.
Nicotine in cigarettes ruin the fibers and connective tissues in your skin, which make skin lose its tautness. This causes your stretch marks to increase with little chances of healing.