Worries over crude prices re-emerged on Monday, pressuring commodities and equities as the S&P 500 slipped following its best week of the year. 

The Dow Jones Industrial Average fell 0.17%, the S&P 500 slid 0.12%, and the Nasdaq was down 0.05%.

Crude oil trading was unpredictable on Monday after Venezuelan Oil Minister Eulogio Del Pino warned that oil prices could drop "to as low as the mid-$20s a barrel" unless the Organization of Petroleum Exporting Countries takes action and cuts output. The group's next meeting is Dec. 4. West Texas Intermediate crude closed 0.4% to settle at $41.75 a barrel.

Oil prices have been under pressure as a supply-demand imbalance has continued. Commodity investors have been bearish as Iranian oil threatened to flood already-oversupplied markets, while global demand, particularly from China, shifted lower. 

"Iran has drawn attention to the planned increase in its production following the lifting of sanctions, though does not itself expect OPEC to decide to change its production policy on 4 December," explained Commerzbank analysts in a note. "Prices are therefore likely to decrease further in the run-up to OPEC's meeting. Given this outlook, financial investors are withdrawing from their oil investments."

Other commodities spent the day in the red, including gold which fell 0.9% to $1,066.70 an ounce, silver which slid 0.4% to $14.07 an ounce, and copper which tumbled 2.5% to $200.80 a pound. 

Pfizer (PFE) - Get Report  was one of the worst performers on the Dow after the drugmaker sealed its acquisition of Dublin-based Allergan (AGN) - Get Report . The $160 billion deal will create the world's biggest drugmaker, as well as the biggest tax inversion in corporate history as Pfizer will move to Ireland.

The S&P 500 wrapped up its best five-day gain of 2015 last week. Benchmark indexes closed last week with gains of more than 3% each after the Federal Reserve made clear a December rate hike was more likely. Instead of pulling the markets lower, investors were relieved at the chance to plan ahead after months of not knowing when the central bank will finally make its move.

There is a "strong case" for hiking rates in December, San Francisco Fed President John Williams said in a speech Saturday. Williams noted that a rate increase is likely if positive economic data continues and as signs that inflation will return to 2% are reinforced.

In earnings news, GameStop (GME) - Get Report plummeted 4.2% following a disappointing third quarter. The video game retailer earned an adjusted 54 cents a share, a nickel below estimate.

Mallinckrodt (MNK) - Get Report shares surged 8.4% after the drugmaker posted a better-than-expected quarter, driven by sales growth in its specialty brands segment, which includes multiple sclerosis drug Acthar.

Sales of existing homes slid 3.4% in October to an annual rate of 5.36 million, according to the National Association of Realtors, in line with analysts' estimates. Sales of previously owned homes remained 3.9% higher than October last year, a sign demand in the housing market remained robust.