Oral caffeine intake is unlikely to be effective for hair loss due to systemic absorption.

“you have so many adenosine receptors throughout your body. Those adenosine receptors, and the parking of caffeine in those adenosine receptors, is the main way in which caffeine exerts its stimulatory effects, making you feel less sleepy. It does that because then adenosine can't have its effects, which are to make you sleepy. Well, those adenosine receptors soak up so much of the caffeine that you would ingest orally that very, very little would make it to the scalp and to the hair follicles at the concentrations that you would want, so that's why you have to rely on the application of these caffeine ointments about three times a week.”