There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the Research
Therapeutic Benefits of Skin Toner
- Skin toner has been found to have therapeutic benefits when used as part of a comprehensive skin care routine 1.
- A study comparing the use of a simple skin care routine (cleanser and day cream) to an advanced routine (cleanser, toner, eye cream, serum, and day and night cream) found that the advanced routine outperformed the simple routine in improving superficial hydration, deep hydration, skin roughness, mean pore area, melanin heterogeneity, and crow's feet wrinkle depth 1.
- Another study found that the use of a toner in combination with a cream led to greater hydration change rate compared to using a cream only, especially for normal skin 2.
- A split-face, randomized, single-blind placebo-controlled study found that a guava toner significantly reduced oiliness of the forehead and nose, making it a potential treatment for oily skin 3.
- While there is limited direct evidence on the therapeutic benefits of skin toner, moisturizers, which are often used in conjunction with toners, have been shown to improve skin conditions such as atopic dermatitis and other chronic skin diseases by restoring the integrity of the stratum corneum and acting as a barrier to water loss 4.
Specific Benefits of Skin Toner
- Improved skin hydration: Skin toner has been found to improve skin hydration, especially when used in combination with a cream 2.
- Reduced oiliness: A guava toner has been found to significantly reduce oiliness of the forehead and nose, making it a potential treatment for oily skin 3.
- Improved skin texture: The use of a toner as part of a comprehensive skin care routine has been found to improve skin roughness, mean pore area, and crow's feet wrinkle depth 1.