How long should it take mebendazole to kill threadworms (pinworms) after taking first dose?

Neurotoxin. Mebendazole acts as a neuro-toxin on the pinworm & kills as it penetrates the stool water where they live.The effect is almost immediate when the drug comes in contact, but may take a few days to saturate all areas. Repeating a dose after a week is a common practice to assure success.