Ivermectin kills parasites within hours — but symptoms persist for weeks. This 2026 guide covers the complete timeline for scabies (12–24 hour kill, 2–4 weeks itch), strongyloidiasis (91.3% cure at 12 months per NIH 2026 study), onchocerciasis, head lice, why symptoms continue after parasites die, when a second dose is needed, and which parasites ivermectin cannot treat.