Retinoids, to which the active ingredient tretinoin belongs, are close relatives of vitamin A. They are important and effective locally and systemically applied drugs for the treatment of skin diseases such as severe acne or psoriasis. Tretinoin is also used in the therapy of a special form of leukaemia.
Tretinoin for hair growth: Doubtful benefits
In reports on its effect on hair growth, tretinoin plays a paradoxical role: On the one hand, diffuse hair loss is one of the known side effects of systemic treatment with the active ingredient  On the other hand, creams containing tretinoin are sometimes used in combination with a minoxidil treatment – obviously in expectation of synergetic effects. This idea was particularly common among dermatologists in the 1980s and 1990s.
In fact, there are a few older scientific publications in which the hypothesis was put forward and examined in smaller studies that a cream containing tretinoin improves the absorption of minoxidil through the scalp (there was talk of a threefold higher absorption )    The keratolytic effect of tretinoin was cited as an explanation for the suspected effect: The substance removes dandruff, cleans clogged pores and could thus make the skin altogether more “permeable”. On the other hand, a comparative study published in 2007 found no difference in the effect of topical minoxidil and minoxidil plus tretinoin on hair growth in androgenetic hair loss.  There are no further clinical application studies; the research community has now turned to newer hypotheses.
Therefore, the benefit of a local application of tretinoin in combination with minoxidil must be considered as if not discredited then at least doubtful. The well-known connection between systemic tretinoin administration and hair loss makes local application even less advisable. Moreover, as the cream irritates the skin, unpleasant skin reactions cannot be excluded .