Caffeine for hair loss?

It would take a lifetime to blog on all the junk that pharmacies knowingly sell as remedies.  I've probably got about half a lifetime left, so I'm only aiming to cover 50%, starting here.

A notable high street pharmacy is selling a shampoo to treat hair loss, which contains caffeine. Coffee for your hair?

People who are interested in transdermal (or percutaneous; through the skin) delivery of drugs use caffeine as a standard to compare the success of different approaches.  Using this as a benchmark for development, several other drugs are now successfully delivered into the blood stream from a patch (eg nicotine).  It's a clever approach, and when done cleverly it means someone wears a patch for a day, rather than taking 3+ pills (and having to remember to).

Caffeine is used experimentally because it's relatively easy to measure in blood, and because it's toxicological and pharmacokinetic (how it is destroyed by the body) properties are known.  So, investigators can see how much caffeine entered the body, and reassure volunteers that the drug that they are absorbing is equivalent to a fraction of a cup of coffee.  Then they can compare their approach - say a new adhesive for a patch, or a spray as an alternative - to previous attempts.  So, there are a large number of studies of the absorption of caffeine by the skin, but that doesn't prove that it does anything to skin, of course.

More recently, several studies have found that a significant amount of a drug that is applied to skin is absorbed via hair follicles.  When hair follicles are blocked, drug absorption is slowed by four-fold.  Caffeine was routinely used in these studies, of course.  So caffeine applied to the skin, gets into the skin, and mostly into hair follicles.  In fact, there's even a study demonstrating that if caffeine is incorporated into a shampoo, it will find its way into the hair follicles within two minutes (Boots' recommended exposure time, interestingly enough).  Caffeine was used in this study, like all the others, because it's the standard for these types of tests.

We're still left with the problem of whether caffeine makes hair grow, or less likely to fall out. This is where two fields collide. Caffeine has a rather complex pharmacological profile, but it is widely held to be an inhibitor of phosophodiesterase enzymes (we don't need to know any more than that for our purposes).  In theory, inhibiting these enzymes would increase hair growth.

And there's one study that shows that caffeine has this effect on isolated human hair follicles.  Yes, just the one.

Now, if you're cynical enough, you can guess how the development team threw that product together.