This is one of the most frequently asked and most poorly answered questions in all of Medicine. I have to confess to pulling out a few of my own when pressed for an answer from a distraught (usually feminine) patient with a perception of thinning locks.
So here goes my usual explanation on the matter of “why is my hair falling out?”:
A head of hair is like a dense forest with 100,000 to 150,000 trees on it. Each tree has a life-cycle of 2-6 years. It grows in spurts from the roots for the majority of that time, (as anyone who uses hair color can confirm) then rests awhile, then falls out and starts over. At any given time around 85% of the trees are in the growth phase, 10-20% are in the rest phase. On any given day a hundred hairs or so at the end of their “rest phase” fall out. A new hair takes its place in the same hair follicle and the cycle stars over. All normal.
The key here is that all the hairs are on a different schedule, so there is usually no overall relative loss of hair over time. But certain events can synchronize the hair cycles causing a large proportion of hairs to reach the fallout phase at relatively the same time. These events can include metabolic changes to the body such as new medications, physical stress like injury or trauma, surgery, rapid weight loss or gain. These events could be hormone changes (like menopause), emotional stress, or allergy. And while there are many diseases and normal hereditary causes of hair loss that can and should be ruled out, the vast majority of cases fall into one or more of the above categories.
One more key thing. A person rarely notices the relative loss of hair until weeks or even months after the precipitating event, so the cause-effect sequence of hair loss is forgotten. The offending event is seldom remembered. Often, by the time a person notices hair loss and someone (a hairdresser, or vitamin store proprietor, perhaps) sells them a shampoo or potion that requires a small bank loan to afford, the original stress is over and the hair cycle is back in sync. So the “hair loss” stops.
The hairdresser gets credit (and profit). Score another victory for snake oil.