By midmorning, shares had dropped 2.1% to $30.11.
The pharmaceuticals company reported net income of $2.3 billion over the three months to March. Excluding one-time items, net income of 57 cents a share beat expectations by 2 cents, according to analysts polled by Thomson Reuters.
Revenue of $11.3 billion fell short of analysts' estimates of $12.1 billion.
The weaker-than-expected report comes days after its latest offer to purchase British drugmaker AstraZeneca for $106 billion was rejected.
Pfizer has recently suffered lower revenues and earnings as key patents expire, giving way to the development of cheaper generic brands. This has been especially true for Pfizer's cholesterol drug Lipitor, the patent for which expired in 2011.