There are several reasons as to why some patients get ulcers after sclerotherapy. It appears that using a solution that is too strong or injected with too much pressure is one of the common causes.
Sometimes ulcers are due to the solution being injected outside the vein. With the newer solutions this is not usually a problem but if hypertonic saline is used then injection outside a vein can often lead to deep ulcers.
Sometimes inexperienced doctors (even under ultrasound guidance) have injected the solution into arteries instead of veins.
Overall the chance of getting ulcers with sclerotherapy is very low but further underlines the importance of having a very experienced doctor treat your veins.