The recommended usage instructions on the product say "In the morning and after every hair wash, use the dosage tip to directly apply product to towel-dried scalp and massage in."

Can Alpecin Caffeine Liquid also be used on dry hair or is it absolutely necessary to wash the hair before application?

Alpecin Caffeine Liquid can also be used on dry hair. It is important that the liquid is applied directly to the scalp with the dosage tip and massaged in. Since the liquid dries quickly on the scalp, you can then style your hair as usual.

Based on the alcohol and menthol content, the liquid may cause a brief refreshing tingle, which subsides quickly. Be sure to avoid contact with the eyes.