A tiny needle is used to cannulate the tiny blood vessels in the skin and a solution is injected into this tiny vessel. This causes the vein wall to become irritated and to stick to itself thereby closing it shut and preventing it from refilling.
It can require multiple injections following which support stockings need to be worn for a week or two.
Treatments may cause a mild to moderate burning sensation immediately following injections similar to a nettle rash in severity.
Veins will commonly appear red for a few days following microsclerotherapy and then fade following the next month or so. Very occasionally the skin can become pigmented or stained.
Information on Microsclerotherapy