Alkaloid found in coffee, tea, and kola nuts. Caffeine is the chief stimulant in beverages such as coffee and tea. It’s often included in skincare products with claims that it will improve the look of cellulite or puffy eyes. Unfortunately, research into caffeine’s effects in this regard are mixed.
Applied to skin, caffeine may have skin-soothing properties.  It can penetrate skin and has a constricting effect, which can help improve the look of redness but also may be sensitizing.
Caffeine’s popularity in products related to cellulite is due to its distant relationship to aminophylline (an ingredient once thought to improve the look of cellulite), which is a modified form of theophylline, and caffeine contains theophylline. 
When it comes to puffy eyes, there is no research indicating caffeine can have any benefit when applied topically. However, caffeine does have potential as an antioxidant, so it isn’t a wasted ingredient in skincare products. [2,3]
- Herman A, Herman A. Caffeine’s mechanisms of action and its cosmetic use. Skin Pharmacol Physiol. 2013;26(1):8-14.
- Shimoda H, Seki E, Michio A. Inhibitory effect of green coffee bean extract on fat accumulation and body weight gain in mice. BMC Complement Altern Med. 2006;6(9):17.
- Kobayashi-Hattori K, Mogi A, Matsumoto Y, Takita T. Effect of caffeine on the body fat and lipid metabolism.