What is the most dangerous complication of injection sclerotherapy for hemorrhoids?

Ulcers. Injection of hemorrhoids is safe in most cases however erosion of the underlying skin may occure following treatment.
Depends. Pain, necrosis, bleeding, infection and only limited long term benefit. If the underlying cause of the flare of the hemorrhoid is not addressed, ie constipation, diarrhea, excessive straining, the hemorrhoid will recur.