Background: Sclerotherapy has been extensively used in the treatment of valvular insufficiency of superficial veins. Although sclerotherapy seems safe, reports of serious adverse events (AE) have been published. This paper aims to review AE of sclerosing agents. Methods: Electronical databases were searched for identifying articles on local, serious and long-term AE of sclerotherapy. Results: Hyperpigmentation and matting are the most often local AE of sclerotherapy. Other local AE include superficial thrombophlevitis, pyoderma gangrenosum, pain, ulcer formation, and hypertrichosis. Local AE can be serious, that is, it can include cutaneous necrosis, intra-arterial injection with subsequent acute ischemia that can lead to amputation, and necrotizing fasciitis. Most data on systemic AE of sclerotherapy are extracted from case series and case reports. Systemic AE include neurological complications, such as ischemic stroke, transient ischemic attack, visual disturbances and cardiac toxicity, that is, myocardial infarction, Takotsubo cardiomyopathy, chest tightness, pulmonary embolism, deep vein thrombosis, septicemia, anaphylaxis. It is difficult to estimate the frequency of serious systemic AE of sclerotherapy. Conclusion: Physicians practicing sclerotherapy should be aware of the possible local and systemic AE of sclerotherapy, inform patients accordingly and be prepared for the appropriate management of the rare but possibly lethal serious AE.