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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 that 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, you should check your scalp type and scalp condition. Depending on your scalp's sensitivity, leaving the product on excessively long can result in temporary redness.