I don't see how that would be a problem: even if it was common, handing out pill to one group but not the other would still lead to one group being more "deworm

I don't see how that would be a problem: even if it was common, handing out pill to one group but not the other would still lead to one group being more "dewormed" than the other, all others things being equal. Anyway, in this case, I do not believe deworming was as common as you assume: > Baseline parasitological surveys indicated that helminth infection rates were over 90%, and over a third had a moderate-heavy infection according to a modified WHO infection criteria (Miguel and Kremer 2004) > Drug take-up rates were high, at approximately 75% in the treatment group, and under 5% in the control group (Miguel and Kremer 2004). 5% of the population taking the drug when >90% should means the drug is not "common".