## 6 Future Perspective  
In order to find a reliable answer to the question about the efficacy of the placenta and its derivatives in hair loss, we postulate that future clinical studies need to address the following:  
* They should be designed as double-blind, randomized trials, controlled by the use of a matched comparator (placebo)--preferably an identical hair product devoid only of the placenta derivative tested, or replaced by minoxidil which is well-established in hair loss trials and routine clinical use.
* Studies should be conducted in patients with well-defined clinical diagnoses (AGA, TE, alopecia areata, etc.). The sizes of study groups should be sufficiently large to produce statistically meaningful data (e.g., hundreds, rather than dozens of patients).
* The studies should involve control groups of gender-, age-, nutrition-, and general health status-matched patients of similar sizes. Controls with the same disease should be preferred over healthy individuals.
* The observation time has to be sufficiently long, considering the velocity of hair growth and duration of the hair cycle phases of interest (we suggest at least 6 months for TE studies, which is within the range of a natural recovery after an acute episode of TE). Longer periods might be necessary for other types of hair loss, or when measuring hair-growth-promoting effects.
* The study outcomes should include objective measures, the number of hairs shed in one standardized combing and trichoscopy seem most suitable. Possible changes in the hair density or hair shaft thickness are likely to become noticeable after several months and would reflect hair growth-promoting, rather than hair-loss-preventing effects.
* When utilizing \"investigator assessment\" as the measure of effect, the assessment method should be standardized and clearly defined. Investigators should be blinded as to which of the compared photographs shows the \"before\" and which is the \"after\" status. Assessment by several investigators (\"voting\") should be preferred over an assessment by a single investigator.
* Patient satisfaction and quality of life are important, therefore, patient-centered measures also should be used; however, their susceptibility to subjective bias should always be kept in mind.
* Tested placenta derivatives should be well defined: A minimal set of descriptors should include the animal source and processing protocol, as well as the concentration and placenta protein content in the final product. Other details should be given whenever possible, e.g., physicochemical properties, pH, spectroscopy, molecular weight, amino acid content, lipids, salt content, etc.  
Aside from clinical efficacy, the safety of the tested products should always be a priority. This topic, however, was beyond the scope of the present study and deserves dedicated research.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142210301/s1](https://www.mdpi.com/article/10.3390/app142210301/s1)]([https://www.mdpi.com/article/10.3390/app142210301/s1](https://www.mdpi.com/article/10.3390/app142210301/s1)), Table S1: A detailed overview of clinical trials in humans on the effectiveness of placenta products in hair loss. Table S2: A detailed overview of animal studies on the influence of placenta products on hair growth.  
Conceptualization, R.S.; methodology, R.S. and E.S.; bibliographic query, E.S.; data extraction, E.S.; data curation, E.S. and R.S.; writing--original draft preparation, E.S.; writing--review and editing, E.S. and R.S.; visualization, E.S.; supervision, R.S.; project administration, R.S.; funding acquisition, R.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Jagiellonian University Medical College, grant number N42/DBS/000445.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflicts of interest.