Takeda Pharmaceutical Co. shareholders' approval of the $62 billion (about ¥7 trillion) acquisition of Shire PLC is good news for its bosses' ambitions to build a global drug giant — but bad news for its credit ratings.

The backing of what will be the largest-ever overseas purchase by a Japanese company paves the way for possible downgrades to the lowest tiers before junk ratings at both S&P Global Ratings and Moody's Investors Service. The changes could come within about a month.

What happens after that depends on how quickly Takeda can reduce its debt burden, including progress in potential noncore asset sales worth up to $10 billion, analysts at the ratings companies say. Takeda pulled off a mammoth dollar- and euro-note bond sale last month, and NLI Research Institute said investors are interested in how much the acquisition ultimately benefits the Japanese drug maker.