Did you take it after a light meal? It can cause stomach upset (like a lot of meds) when it's taken on empty stomach.
It's recommended to take it after a light meal such as breakfast. It must be taken for seven days straight to prevent reinfection.
I never had much luck with Niclosamide myself, but that could have been due to reinfection. Dwarfs are highly infectious and spread easily from person to person. Your whole family or whoever you live with should be treated as well to avoid reinfection.