Here's the material that I promised earlier. It's from the following study:
"Relationship between Contact Time of Applied Dose and Percutaneous Absorption of Minoxidil from a Topical Solution", James J. Ferry, Judy H. Shepard, and Gregory J. Szpunar, Journal of Pharmaceutical Sciences Vol. 79, No. 6, June 1990.
Received April 19, 1989, from the
Clinical Pharmacokinetics Unit, The Upjohn Company, Kalamazoo, MI 49001.
Here's the insightful paragraph near the end in the DISCUSSION section which tells the story about what happens when you apply Rogaine to your scalp:
"The topical solution used in this study consists of a mixture of ethanol:water

ropylene glycol in relative proportion (v/v) of 60:20:20. Ethanol and water are extremely volatile relative to propylene glycol and application of this vehicle to the scalp likely results quickly in a solution of minoxidil in propylene glycol approximately equivalent to the solubility maximum of 100 mg/mL. Percutaneous absorption of minoxidil after topical administration presumably occurs by diffusion out of this saturated propylene glycol solution."
So in other words, when you apply 5% Rogaine to your scalp, the alcohol and water quickly evaporate, leaving behind a saturated solution of minoxidil in propylene glycol. And when you apply 2% Rogaine, the same thing happens: the alcohol and water quickly evaporate, leaving behind a saturated solution of minoxidil in propylene glycol. The only difference between those two is just the QUANTITY of the saturated propylene glycol that they leave behind. But applying 2.5 mL of 2% would produce the same quantity of that saturated propylene glycol as applying 1 mL of 5%. So in that important sense, 2% and 5% versions of Rogaine are
equivalent: they BOTH turn into the same thing (albeit different quantities) when you apply them to your scalp.