One of the first articles regarding the use of PRP in hair diseases was by Uebel et al. , in which the authors performed a study in patients undergoing hair transplant surgery. The hair follicular units were embedded in PRP before transplantation. The authors found an improvement of hair growth and an increase in follicular density. The GFs could act upon dermic papilla (DP), leading to intense neovascularization and the progression of new hairs to the anagen phase.
Since then, PRP has started to be considered as a potential therapeutic tool for promoting hair growth, although the precise mechanism by which PRP promotes hair growth is not fully understood .
In recent years, several articles [7, 17, 19, 27,28,29,30,31,32] have been published that refer to the positive effect of the treatment in AGA (Table 2).
Alves and Grimalt  performed a clinical trial to access the efficacy of PRP. The results of the randomized placebo-controlled, double-blinded, half-head trial revealed that the administration of PRP led to a significant increase in the mean total hair density, the total terminal hair density, and the number of anagen hairs after 3 months and after 6 months in comparison to the baseline. Also, the anagen/telogen ratio (%) increased, which led to faster telogen-to-anagen transition, as described by Li et al. .
Maria-Angeliki et al.  published a systematic review to analyze PRP mechanism of action, preparation methods, and therapeutic potential in patients with non-cicatricial alopecia. Among the 14 studies included in their systematic review, they found that PRP could be a possible useful treatment for non-cicatricial alopecia. In most studies, activation and preparation methods were not mentioned, and no standard protocol was employed regarding the frequency of PRP applications.
In 2019, a literature search combined with meta-analysis  was used to calculate the overall standardized mean difference in hair density in patients treated with PRP injections in comparison with baseline and placebo treatment. Ten studies in which parameters were considered more scientifically rigorous were included. The results of meta-analysis favor the treatment of PRP when compared with placebo and baseline hair restoration parameters (especially hair density) in AGA monotherapy or adjunct therapy.
Patient characteristics may also influence the results of PRP treatment as stated before by Alves and Grimalt , in which they demonstrate a statistically significant association between hair density and patients below 40 years with positive family history of AGA and more than 10 years of duration of the disease. In addition, there was a correlation between anagen hairs and patients older than 40 years and beginning of AGA with age superior to 25 years.
Many of the articles published focus on patients who received treatment with PRP alone. Patients who received topical and/or systemic treatments for AGA in the previous 12 months were typically excluded. Considering that most patients with AGA observed in our clinical practice are undergoing therapy for their alopecia, it is postulated that PRP used in combination could help to achieve better results, especially in patients whose condition appears to stabilized or the improvement is slow.
A clinical trial was designed to assess the efficacy of PRP in combination with 5% minoxidil topical solution and 1 mg finasteride orally administered for the treatment of AGA. The present study included 24 patients with AGA. Patients were instructed to maintain their usual AGA treatment throughout the protocol or were excluded from the study. They received a cycle of three treatments (at a 1-month interval) with PRP on half of the head and saline solution (placebo) on the other half. Follow-up took place after 6 months. On the basis of the obtained data, treatment with PRP in combination with a concomitant medication (topically administered minoxidil or orally administered finasteride) significantly increased the number of hairs, hair density, terminal hair density, and the number of anagen hairs in comparison to baseline and the control side. In addition, patients who received topically administered minoxidil along with PRP showed a greater improvement.
A randomized, controlled, crossover pilot study was performed  in order to evaluate the efficacy of PRP in the treatment of AGA when compared with topically administered minoxidil. Twenty female patients were selected and divided into two arms: arm A received PRP injections every 4 weeks for a total of three treatments; then patients underwent an 8-week washout. Thereafter, they received crossover treatment with minoxidil, applying the 5% foam once daily, for a total of 12 weeks. Patients in arm B were randomized reversed in sequence, first using minoxidil for 12 weeks, followed by an 8-week washout, and then performing the three treatments with PRP. According to the authors, the results of the study suggest that PRP is effective as a treatment for hair regrowth in female AGA, although perhaps not as effective as minoxidil, despite the quality-of-life questionnaire showing a better response after PRP use versus minoxidil.
Lachgar et al.  demonstrated that minoxidil stimulates the production of GFs, such as VEGF, in cultured DP cells, and that the effect might promote hair growth. This upregulation of VEGF helps maintain DP vasculature and hair growth . PRP also produces different GFs, leading to intense neovascularization  and the increased proliferation of human DP cells . This effect might support the hypothesis that PRP potentiates the effect of minoxidil by promoting the anagen phase and delaying the initiation of the catagen stage. Although more studies are needed and the mechanism is not fully understood, the combination of topically administered minoxidil and PRP appears to lead to a stronger improvement of regrowth.
Thus, one might consider PRP to be safe and effective when used concurrently with the patient’s current medication.
Regarding the protocol used in PRP there is no standardization. According to the literature and personal experience, a minimum of three sessions should be performed 1 month apart. Then, at least two or three more sessions should be performed after 6 months to maintain the regrowth .
The available articles concerning PRP use and other types of alopecia are sparse. Only a few articles discuss PRP efficacy in patients with alopecia areata (AA). AA is a common non-scarring type of alopecia, which has different clinical presentations, classified according to the hair loss pattern or extent . In some cases, AA is associated with other autoimmune diseases such as vitiligo, atopic dermatitis, diabetes, hypothyroidism, and pernicious anemia [39,40,41].
There is an unpredictable course of AA regarding the treatment options. Spontaneous regrowth sometimes occurs over several months.
The most common treatments in patients with AA are topical or intralesional corticosteroids (triamcinolone acetonide) and systemic corticosteroids (continuous or as pulse therapy) [42,43,44]. Several treatment modalities include topically administered minoxidil, topical immunotherapy (squaric acid dibutylester), anthralin, phototherapy, immunosuppressants, and immunomodulators .
Trink et al.  published the first article regarding treatment with PRP in patients with AA. In 45 randomized patients, some received intralesional injections of PRP, triamcinolone acetonide (TrA), or placebo in one half of their scalp. The other half of the scalp was not treated. After three treatments of PRP in the affected areas, the authors found that the administration of PRP increased hair regrowth in comparison to triamcinolone acetonide or placebo. Also, there was a decreased of the burning or itching sensation after treatment with PRP.
Another article  regarding PRP and AA included 20 patients for whom different therapies for AA had proved unsuccessful in the previous 2 years. The patients received a total of six PRP sessions at 1-month intervals. According to the authors, after 1 year of follow-up, only one of the 20 patients had a relapse.
Albalat et al.  performed a study to compare the safety and efficacy of PRP versus the injection of intralesional corticosteroids (ILCs) in 80 selected patients with AA. The patients were classified randomly into two groups: group I (n = 40) received from three to five sessions of ILCs, one session every 2 weeks; and group II (n = 40) received one session of PRP every 2 weeks, for three to five sessions. The authors found significant regrowth of pigmented hair and decrease of dystrophic hair in both groups. The difference between both groups was insignificant, although more relapse was observed with ILCs.
There are many subtypes of AA, such ophiasis form, which affects the occipital and parietal scalp. This form of alopecia usually is more resistant to treatment. Jeff Donovan described a case report  of a patient with ophiasis-type alopecia areata, resistant to minoxidil, topical steroids, and three treatment sessions of ILC injections. In this case, after 2 years of no response, the authors administered autologous PRP in the affected areas. PRP led to hair regrowth in the first month with “robust regrowth of hairs measuring 2.8 cm by month 3”. The effectiveness of PRP specifically in ophiasis warrants further study.
Another type of alopecia areata is alopecia areata totalis. Khademi et al.  published a study with ten patients with clinical diagnosis of AA totalis for at least 3 years who had not received any therapy within 3 months before the beginning of the study. The patients received one single intradermal injection of PRP on the scalp. After 4 months of follow-up, no hair regrowth was seen in eight patients and in two patients only less than 10% hair regrowth was observed. Overall, no significant effect was found for PRP on hair regrowth.
Some studies demonstrated that PRP is effective for the treatment of patients with AA  and that PRP could be a valid treatment option for patients with AA, but not for AA totalis, in which no regrowth was noted.