My last (5th) cleanse I had to stop after 2 1/2 weeks.
I still felt good and that the cleanse helped a great deal despite having to stop it early.

From what HW says it is the constant stopping and starting over a long period of time that creates stronger parasites, not a once or twice thing.

I don't think the HW killed what it could have with the full 4 weeks. But I found that it did help a great deal.

21 days is fine :)