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 activate the hair roots.
Leaving the product on for a longer period of time also boosts caffeine absorption in the scalp. A study by the Charité University Clinic in Berlin has shown that absorption of the active ingredient increases for a period of up to 30 minutes when the product is left on the scalp. After that, penetration via the skin surrounding the follicles has increased to such an extent that additional caffeine availability is ensured for up to about eight hours. If you leave the product on the scalp for longer than 2 minutes, depending on your scalp's sensitivity, this can cause temporary redness.