What is Polidocanol? Is it better than sclerotherapy for spider veins? Does it cause "shadowing"? Is it available in the US?

Polidocanol is a sclerosant solution classified as a chemical detergent.  It can be used in foam solution sclerotherapy but is not FDA approved in the United States at this time.  There are alternative solutions in the United States and these can be administered by a board certified vein specialist.

There are several ways to treat spider veins in the legs.
The most common, and still considered the 'gold standard', is with sclerotherapy.
Laser can also be a good option if the vessels are so tiny that it is nearly impossible to cannulate them with a needle and sometimes as an option when treating large, bulging vessels by entering inside of the vessel and sealing them from within.
There are several sclerosants available and all will adequately do the job. Sometimes it is just personal preference from the clinician.
Polydoconol can be obtained and used in the U.S. This is actually the choice method in our office. The average normal concentrations can be 0.25% to a 1.0% solution, depending on vessel size being treated. The shadowing that you are referring to is actually an increase in the pigment of the skin that can occur after the procedure. One significant way to minimize this is to be as sun-free as possible prior to the treatment. Bleaching creams can also help to reduce this effect. By applying the cream prior to the procedure and afterwards can help reduce the chance that the pigment cells will be activated as strongly, and thereby reduces the 'shadowing effect'.

Polidocanol is a solution used for the vein injections called sclerotherapy. It is not FDA approved, but so far it is one of the best agents to treat spider veins or telangectasias. During the procedure a solution which is irritating to the vein is injected into the vessel with a very fine needle causing irritation and inflammation in the wall of the vein resulting in scarring of the vessel. The results of the sclerotherapy depend also on how well patient is wearing the compression stockings, which are prescribed by the physician.