Walmart Inc. dropped to the bottom of the S&P 500 Consumer Staples index in December 2020 after the U.S. Justice Department sued the big-box retailer, alleging that it contributed to the growth of the opioid crisis in the country.

The S&P 500 Consumer Staples index rose 1.8% during the month, lagging the wider S&P 500's 3.8% growth, according to data compiled by S&P Global Market Intelligence. For the second consecutive month, all 11 sectors of the S&P 500 posted gains.

Of the 33 constituents of the consumer staples index, 16 recorded share-price increases during the month.

Walmart's shares fell 5.3% in December. The Justice Department on Dec. 22 accused the company's pharmacies of unlawfully distributing opioids and said it would seek penalties that could total billions of dollars. Walmart also plans to lay off 1,241 workers across two states on Jan. 31 as it focuses on its omnichannel strategy, according to a Dec. 1 report.

The Kroger Co. was the second-worst performing consumer staples stock with a 3.8% decline during the month. The grocer's shares fell 4% on Dec. 3 after reporting fiscal third-quarter sales that missed expectations.

Food company Campbell Soup Co.'s shares declined 3.3% in December despite reporting fiscal first-quarter results that beat analyst estimates.

Meanwhile, Philip Morris International Inc. was the best-performing stock on the S&P 500 Consumer Staples index in December with a 10.9% gain. On Dec. 2, the tobacco company said it expects its 2020 earnings to come in at the top end of its guidance. The company also named a new CEO and executive chairman.

Monster Beverage Corp. registered a 9.1% increase during the month as several analysts, including those at Wells Fargo and Consumer Edge Research, raised their target price for the company. Lamb Weston Holdings Inc. was the third-best performing consumer staples stock. The frozen potato snacks producer on Dec. 17 increased its quarterly dividend.

Media ratings and analytics group Nielsen Holdings PLC was the best performer on the S&P 500 with a 29.1% gain in December. The company's shares jumped 9% on Dec. 9 after it unveiled a plan to replace the way it measures audiences to incorporate digital viewing.

Pest and termite control service provider Rollins Inc. the worst-performing stock on the S&P 500 as its shares fell 31.7% during the month.