The severe decline for shares of Express Scripts this month is a buying opportunity, according to Citi, which upgraded the company.

The St. Louis company’s stock price closed at $50.05 on Wednesday and has plunged more than 20 percent since Express Scripts leaders said earlier on Nov. 6 that Wall Street expectations for 2013 earnings were “overly aggressive.”

In contrast to that drop, Citi analyst George Hill noted that the Standard & Poor’s 500 index has fallen 4 percent, while shares of Express Scripts’ biggest competitors have dropped only 2 percent. Hill said in a Thursday morning research note that most investor concerns are now factored into the company’s stock price, which reached an attractive level “with a compelling risk/reward profile should the company deliver on recently lowered investor expectations.”

Hill raised his rating on the stock to “buy” from “neutral.”

Shares rose 33 cents to $50.38 before the opening bell.

Express Scripts runs prescription drug plans for employers, insurers and other customers as the nation’s largest pharmacy benefits manager, or PBM. It processes mail-order prescriptions and handle bills for prescriptions filled at retail pharmacies. Express Scripts also announced earlier this month third quarter earnings that beat Wall Street expectations.

But the company said 2013 results will be hurt by factors like high unemployment.

Despite the recent plunge, the company’s share price is still up 12 percent so far this year.

Earlier in the year, Express Scripts Holding Co. completed a $29.1 billion acquisition of Medco Health Solutions.