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The levels you get from topical use are much lower than the oral dose and are 2 ng/mL or lower (which = 0.0095 uMol/L) even at the 5% dose, and even with multiple daily applications. [Ref: AHFS Monograph, Medscape on Minoxidil topical pharmacokinetics]

Meanwhile, the minimum dose required to suppress lysyl hydroxylase in fibroblasts (the cells and enzyme it inhibits in collagen synthesis) is 25 uMol/L (Br J Derm 2004; 150, 186-194) or over 2600 times the blood concentration you develop.

Even very rare individuals who have abnormally high absorption through skin found to have 30ng/mL in the blood would still need 173 times the concentration they have to inhibit collagen synthesis in skin fibroblasts. And remember, skin concentrations outside of the area of application don?t have the level of drug found in the blood.

This is further evidenced by the fact that when dermatologists initially saw a few studies noting such collagen effects, it was theorized that direct application of minoxidil onto skin might help prevent scarring from collagen formation. Minoxidil failed to prevent such scarring from collagen formation, even where minoxidil was directly applied, because in part it is believed to inhibit a version of the enzyme which is not as critical to collagen cross linking in the first place. (http://www.ncbi.nlm.nih.gov/pubmed/15908192)