Standard & Poor's downgraded Pfizer Inc.’s rating from AAA to AA, following the Pfizer-Wyeth deal.

Recently, Pfizer has bought Wyeth in a deal worth about $67 billion, which will require the drug-maker to take-on new debt of $22.5 billion.

Speaking on the move, S&P stated, “Significant additional borrowings needed to fund the acquisition.”

Standard & Poor's also lowered Pfizer’s stock rating from AA+ to A+.

Earlier, Moody's Investors Service downgraded its rating from Aa2 to A1, while Fitch Ratings lowered the Pfizer from AA to AA-.

In the recent trading, shares in Pfizer gained 0.8 per cent to close at $17.80-a-share.