Skin along the bikini line, the inner thigh and under your arms has more pigment-producing cells than the skin around it to begin with. Every shave, every wax, every ingrown hair, every seam that rubs, each one irritates those cells, and irritated cells make more pigment. A decade of that builds up quietly, which is why one day you notice it and assume it appeared out of nowhere.
This one works on the pigment itself. Tranexamic acid interrupts the signal that turns irritation into pigment. Niacinamide stops the pigment already made from reaching the surface. Glutathione and acetyl glucosamine work further back in the same pathway. Four points, one problem.
The dark patch gets visibly less dark. Then less again. Until the crease stops being a different shade to your leg.
One honest thing. It won't change your baseline tone. The skin in the intimate area is naturally deeper than the rest of your body in most people after puberty, and nothing topical alters that. What it clears is the darkness that was added on top. That is the part you noticed, and that is the part that goes.