Pfizer, the maker of Viagra and Lipitor, has struck a deal to buy Botox maker and Irish-based Allergan in a transaction valued at about $160 billion. The complex deal, the biggest ever in the healthcare sector, will allow Pfizer to shift its legal base to Ireland in a so-called inversion that will reduce its corporate tax rate. Pfizer has had a significant manufacturing presence in Ireland for years. Pictured is the Pfizer facility in Dun Laoghaire, Co. Dublin. RollingNews.ie photo.