Publisher Summary This chapter discusses adverse effects of antihelminthic drugs. Adverse effects of ivermectin are mainly attributed to the decay of dying parasites and not to intrinsic effects of the drug itself. Praziquantel is usually considered to be the most effective drug in the treatment of all types of human schistosomiasis. It is effective and well tolerated in single-dose treatment, although transient adverse effects, consisting of abdominal discomfort, bloody diarrhea, and allergic reactions (urticaria and edema) often occur, especially in the more heavily infected individuals. The systemic adverse effects of treatment with suramin include nasal stuffiness, nausea, fever with chills, weakness, or urticaria. Suramin causes a unique pattern of extensive adverse effects, consisting of a syndrome of general malaise, anorexia, skin rash, edema, increased serum creatinine concentrations, proteinuria, increased transaminase activities, hematological abnormalities such as leukopenia, anemia, thrombocytopenia, rarely irreversible adrenal insufficiency, and neurological complications ranging from paresthesia to motor disturbances and a Guillain–Barrdé like syndrome.