This essence is actually the main vitamin C, and its concentration is very high, reaching 20%, so the brightening and whitening effect on the skin can be seen almost immediately, which may be the reason why it sells first. But it should be noted that 20% concentration of vitamin C is too irritating to some people. It is suggested to use other products with low vitamin C concentration (such as Duke's 10% essence) to gradually establish skin tolerance. OZ Naturals, the essence, works better with nicotinamide products. It needs to be stored away from light, and it should be used up as soon as possible after opening to avoid oxidation.