The Radio 4 “Best Thing Since Sliced Bread” radio show and podcast with Greg Foot has looked at the claims made by the makers of Caffeine shampoos with the Queens hairdresser, Ian Carmichael and consultant dermatologist, Dr Paul Farrant. 
Some lab based in vitro studies of hair follicle cultivation in 2007 showed that testosterone negatively impacted on hair growth and adding caffeine to the culture solution reversed the negative impact. Hair follicles soaked in a caffeine solution increases follicle growth but how does that translate to real life and commercial available products like caffeine shampoos?
You’ll have to listen to the programme or download the podcast to listen to the views of the experts. Spoiler alert – there is promise to caffeine from a biological perspective but the published studies in humans using either shampoo or lotion are of very poor design, small in number of subjects and use very questionable outcome measures (the hair pull test – a test hard to standardise and therefore not used in drug based clinical research). In short, the evidence they work is not there yet to have confidence that they really do “stimulate” or “energise” your roots!