How long can it take to have a reaction to Sclerotherapy?

The
reaction described 18 days after treatment is unlikely to be directly due to
the sclerosing solution used for
Sclerotherapy. Are compression stockings
being worn after treatment? These
contain latex, which may be the cause of the reaction described. Are any creams or lotions being applied to
the leg? These may also cause such a
reaction if the skin is irritated by or allergic to an ingredient in the
product.Infections usually don’t itch
and don’t usually appear as ‘mosquito bite like bumps’, but a physician
evaluation would be necessary to rule out infection.

A true allergic reaction to a sclerosant solution is most likely, albeit a very rare event. The sclerosant solution is long gone and out of your system within hours of the injection session. You may have something else going on like phlebitis (inflammation of the veins) or more likely, gravitational eczema - also known as stasis dermatitis or venous eczema. See a dermatologist. 

It would be most unusual to have a reaction almost 3 weeks following sclerotherapy.  Could there be another reason for the reaction? Try local care with caladryl lotion or oral Benadryl and follow up with your treating physician.