Try levamisole 150 mg once a week for 4 weeks, then give it a rest and
see what happens.
Also, what do you mean by threadworms? Is it pinworms or strongyloides?
Pinworms are often refered to as threadworms. If it's pinworms you
may want to check your environment. If you work with children, you
may be getting it from them. Or the eggs may be floating in the dusty
air in your own place. You may inhale them without being aware.
Watch out for the dryers in the public laundromats. Their exhaust can
spread pinworm eggs.
A friend of mine had a problem with strongyloides. He tried
everything until he took livamisole. That ended it for him.