Johnson & Johnson today officially launches its over-the-counter version of Zyrtec, the leading prescription allergy drug in the US last year, with sales of $1.57 billion. Schering-Plough, which sells Claritin, the leading OTC allergy product, is responding by running TV ads that flag a warning on Zyrtec’s label about drowsiness, The Star-Ledger of New Jersey reports.

Meanwhile, the FDA recently approved generic forms of Zyrtec, which means some pharmacies could market private-label versions of the antihistamine. “Now it’s all about the marketing and widespread promotion,” Laura Mahecha, health-care industry manager for the Kline Group, a consulting firm that tracks the OTC biz, tells the paper.

And what is J&J’s idea of marketing. The health care giant has temporarily erected an 8,100-square-foot dome in New York’s Central Park called the “Spring Wonderland Dome.” Through Sunday, visitors can get a free cup of hot chocolate and participate in activities that call to mind spring - and, not so coincidentally, allergies - such as gardening and pet training. Duane-Reade, one of the city’s best-known drugstore chains, will be there, too.

Coincidentally, Schering-Plough this past week won FDA approval to change Claritin’s packaging label so it more closely matches Zyrtec in the range of allergies it treats. Last November, by the way, the drugmaker sponsored “Drowsy Driving Prevention Week” with the National Sleep Foundation, and took a swipe at Zyrtec in a press release announcing the event. Schering-Plough ceo Fred Hassan isn’t fazed. “If we find there is some kind of share attack on our brand, we will certainly defend our brand.” Just like he did with Vytorin, no doubt.