Apart from Alpecin Caffeine Liquid, can I use other products such as gel or hair spray?

Yes, after massaging your scalp with Alpecin Caffeine Liquid, wait about 3 minutes until the product is absorbed before using your styling product. The recommended usage instructions on the product state: "In the morning and after every hair wash, use the dosage tip to directly apply product to towel-dried scalp and massage in."