Pfizer, the US's biggest drugsmaker, is in talks with rival Allergan about a buy-out - a deal that would be the biggest in the drugs industry's history. Buying Allergan would give Pfizer a way to move to a low-tax address abroad, and gain valuable speciality drugs such as Allergan's Botox.Pfizer was interested only in a friendly transaction, said a source.Allergan shares rose 11% to $319.50 in New York trading before the market opened. Pfizer shares rose 2.4% to $36.30.Purchasing Allergan, which has its legal domicile in Dublin, and a market capitalisation of $113-billion (about R1.52-trillion), might let Pfizer relocate outside the US for tax purposes through what's know as a tax inversion.The move requires a large foreign target in order to clear US tax rules and was one of the reasons Pfizer tried to acquire Astra Zeneca last year.Pfizer and Allergan declined to comment.Pfizer CE Ian Read said this week that he was looking at deals, as the company often does, and was ready to be "bold"."This management team is not afraid of looking at opportunities," Read said on a conference call on Tuesday before the report of talks with Allergan."When we make our decision as to what is the best way of enhancing value, we will move." ..