I'm not finding anything online that says nettles contain Anticholinergic alkaloids. Also the danger from the specific antihistamines that were studied was in people who used them in higher doses and for prolonged periods. The increased risk was only found in people 65 and over who took these medicines at the equivalent of once every day for more than three years. No link was found at lower levels. And the antihistamines involved were only one, older class known to cause drowsiness.