Is it true that excessive sebum production reduces the effectiveness of minoxidil? Will minoxidil work if scalp is oily?

As  with any topical formulation, absorption of the active ingredient is a determinant in the effectiveness of the product. When one has excessive sebum, it is possible that the absorption is diminished. Minoxidil foam has a formulation that should penetrate sebum. You may want to try washing your hair with a dandruff shampoo and drying it prior to application of the minoxidil. Should it still not be effective, oral minoxidil produces results without necessitating absorption through the scalp.

Minoxidil is least effective when it is not used. Oil may reduce effects slightly but only slightly. You might consider treatments for seborrheic dermatitis to counteract oiliness. Be sure to see your physician to review such treatments.

The best way to use minoxidil is after a hot shower with a good shampoo at night,  That will take away the sebum and get the minoxidil to be absorbed best