Is treatment necessary?

Varicose veins and spider veins are always abnormal and will not disappear without treatment.  Left unchecked varicose veins can lead to unpleasant and even serious complications as skin discoloration and sores, bleeding, ankle swelling and phlebitis.  Spider veins, not associated with varicose veins, will likely remain only a cosmetic problem although some patients complain bitterly of discomfort for no other apparent reason.  While many persons chose not to treat varicose veins, especially when they are not causing pain or discomfort, in the minority of cases that complications develop, treating the varicose veins after the fact cannot reverse the complications and therefore comes too late.

The agents used to inject veins during sclerotherapy are drugs; can one have an allergic reaction while having these injections?

Allergic reaction during sclerotherapy is a very rare occurrence because the drug is generally trapped in the local area of injection. In extremely rare situations, respiratory and vascular collapse (anaphylaxis) can occur, which is life-threatening. If it does occur, your doctor has methods to manage the symptoms, which is most commonly quite effective.