Port wine stain on cheek - can minoxidil cause it to increase in size?

That’s a very insightful and important question — and I completely understand your concern.
A port-wine stain is a type of vascular birthmark caused by dilated capillaries (small blood vessels) in the skin. On the other hand, minoxidil works by opening potassium channels, which helps dilate blood vessels and improve circulation in the area where it’s applied — primarily to stimulate hair growth.
In theory, because both involve blood vessels, it’s understandable to worry that minoxidil might cause a port-wine stain to become more prominent or increase in size. However, in practice, there’s no strong evidence or clinical data showing that topical minoxidil enlarges or darkens port-wine stains. Most of its effects are localized to the hair follicles rather than causing significant structural changes in the deeper vascular layers of the skin.
That said, everyone’s skin can respond differently — especially when a vascular malformation is present. If you notice any redness, darkening, or change in texture of the stain after starting minoxidil, it’s best to stop using it and monitor for improvement.
In summary, while the risk is likely low, it’s reasonable to be cautious and closely observe your skin for any changes when using minoxidil over or near a port-wine stain.

We do not have any evidence that patients using minoxidil on the scalp for hair issues experience enlargement of port wine stains. It is important to be aware that Vasodilators do not typically increase the appearance of port wine stains because the blood vessels in port wine stains exhibit a reduced response to both vasodilating and vasoconstricting stimuli compared to skin unaffected by port wine staiins. Be sure to review these issues with your doctors and should you wish to start, simply ttakr photographs every 3 months for 9 months to monitor. It would not be likely there is any difference.