There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the Research
Benefits of Skin Toner
- Skin toner can be beneficial in reducing oiliness of the skin, as shown in a study where guava toner significantly reduced oiliness of the forehead and nose compared to a base toner 1.
- The use of a skin care routine that includes a toner can provide clinical benefits, such as improving superficial hydration, deep hydration, skin roughness, and reducing melanin heterogeneity and crow's feet wrinkle depth 2.
- Toner can be used in combination with other moisturizers, such as cream, to achieve greater hydration rates, especially for normal skin 3.
Comparison with Other Moisturizers
- The application of cream only can be an effective and brief way to achieve favorable moisturization, especially for dry skin 3.
- Repeated application of toner can be efficacious for skin hydration, especially when used in combination with cream 3.
Bioactive Ingredients in Toners
- Toners can contain bioactive ingredients, such as guava extract, that have been shown to provide dermatologic benefits, including anti-sebum efficacy 1.
- Other bioactive ingredients, such as those found in Korean cosmeceuticals, can provide benefits for skin rejuvenation, photoprotection, wound healing, and more 4.