In principle, the longer you leave the shampoo on the scalp, the more intensive the effect of the caffeine. However, this may increase the risk of scalp redness especially for sensitive scalps. Therefore, you should pay close attention to how your scalp reacts, when you leave the product on longer. Normally, 2 to 5 minutes should be enough to stimulate the hair.
Leaving the product on for a longer period of time also boosts the caffeine’s effect on the scalp. If the product is left on the scalp for longer than 2 minutes, then the consumer should check his scalp type and scalp condition. Depending on the scalp's sensitivity, leaving the product on excessively long can result in temporary redness.
With regard to the hair wash, the quantity of caffeine applied per usage is about one-fifth of a cup of coffee or 50 ml of Coca-Cola. This is completely safe, even for people who react sensitively to caffeine.