The British Journal of Dermatology (2014 May 16. doi: 10.1111/bjd.13114.) The authors were Fischer TW1, Herczeg-Lisztes E, Funk W, Zillikens D, Bíró T, Paus R..
The study suggested that caffeine promoted hair growth. Human male and female hair shafts were cultured in the presence of testosterone. Many factors were studied. “Caffeine enhanced hair shaft elongation, prolonged anagen duration and stimulated hair matrix keratinocyte proliferation”. Females were more sensitive to caffeine than males. From the abstract I read, it was not clear that drinking caffeine in your coffee or tea worked as the study was on human hair shafts out of the body.
Dr. Jerry Cooley (not a researcher in this study but a practicing hair transplant surgeon) reported the use of topical caffeine products such as “shampoos like Hair Surge or topical leave ins like Nioxin Diamax, which has been rebranded as Pantene Age Defy and Head &Shoulders Advanced Thickening Tonic;” however, I have no experience with these for promoting hair growth. What is needed is a real human clinical trial before drawing conclusions that there is some value in caffeine in some form.