I have had several dermatologists ask me about the value of applying mayonnaise to the scalp in patients with severe seborrheic dermatitis and psoriasis. I think this home remedy has been popularized in several blogs and consumer publications. I do know that mayonnaise is very difficult to shampoo out of the hair and the patient who uses this treatment will have greasy hair for days. I believe the rationale for this treatment is that the oily nature of the mayonnaise will soften the retained scalp scale and allow it to desquamate more readily. Indeed, there is a scalp treatment for seborrheic dermatitis and psoriasis that dissolves a medium potency corticosteroid in peanut oil for much the same reason.
I personally would not recommend mayonnaise to the scalp, but patients who cannot afford medication could use mineral oil. Pure mineral oil does not support bacterial growth and develop an odor while mayonnaise or other vegetable oils can become rancid. Mineral oil should not be used, however, in children or the elderly where inhalation may be a problem resulting in aspiration pneumonia.