Like Tom Cruise in “Top Gun”, the mergers and acquisitions boom has entered the danger zone with two mega-deals in America. Actavis, a drugs firm, will pay $66 billion for its rival Allergan, the maker of Botox, which has been in play for months. The bill is queasily high: 35 times Allergan’s earnings and 97% more than its price at the end of 2013. Expected gains equivalent to almost half of Allergan’s cost base look unrealistic, and the combined firm will be heavily indebted. Investors don’t seem to care for now and have sent both firms’ shares higher. They were less keen on Halliburton’s decision to buy Baker Hughes for $38 billion. A union of the two oil-services firms makes sense, particularly as lower oil prices lead to budget cuts in the energy industry. But antitrust regulators may object. As the M&A boom intensifies, valuations are creeping up to record, vertiginous highs.