The Dow Jones Industrial Average (^DJI +1.08%) index edged 3.9% higher in May, with 70% of its 30 constituent stocks ending the month in positive territory. However, the two worst-performing Dow stocks in May -- both from the healthcare sector -- kept the index's rally in check, with one of them plunging over 25%.

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1. UnitedHealth Group

Shares of UnitedHealth Group (UNH +2.79%) crashed 26.6% in May after multiple negative developments sent shockwaves through the investing community. In mid-May, UnitedHealth suddenly replaced its CEO, Andrew Witty, with former CEO Stephen J. Hemsley with immediate effect and suspended its earnings outlook for the full year because of a surge in medical costs.

In between, The Wall Street Journal reported a criminal investigation against UnitedHealth by the Department of Justice for a "possible medicare fraud." Days later, The Guardian released a scathing report alleging UnitedHealth put patients' health at risk by paying secret bonuses to nursing homes to cut hospital transfers.

UNH - UnitedHealth Group

$406.67+$11.05 (+2.79%)

NYSE · As of 4:00 PM ET 10/15/26

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Key data points

Market capMarket cap calculated using publicly traded shares outstanding only. Does not include unlisted, private, or dual-class non-traded shares. Implied market cap may vary.$365.02B

Day's range$400.00 - $414.12

52wk range$255.97 - $461.62

Volume10.2M

Avg volume5.2M

Dividend yield2.23%

For now, UnitedHealth expects to "return to growth" in 2026 and has sued The Guardian for defamation. Regaining investor confidence, however, may not be easy. The stock is down 38% so far this year, as of this writing.

2. Merck

Merck (MRK -1.48%) stock lost 9.8% in May and plunged to its 52-week low of $73.31 a share after President Donald Trump signed an executive order directing drugmakers in the U.S. to cut the prices of prescription drugs. This comes at a time when Merck already expects tariffs to add $200 million to its costs this year.

MRK - Merck

$145.19-$2.18 (-1.48%)

NYSE · As of 4:00 PM ET 10/15/26

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Key data points

Market capMarket cap calculated using publicly traded shares outstanding only. Does not include unlisted, private, or dual-class non-traded shares. Implied market cap may vary.$358.21B

Day's range$144.91 - $152.34

52wk range$82.01 - $156.92

Volume13.9M

Avg volume9.9M

Gross margin72.43%

Dividend yield2.34%

Investors are also worried about Merck's future once its blockbuster oncology drug Keytruda loses patent exclusivity. In mid-May, analysts at Citi slashed Merck stock's price target to $84 per share from $115 per share citing the pharmaceutical company's growth challenges and a "lack of urgency" to develop a business that could soften the impact of a potential loss of up to 20% of sales for Keytruda once its patent expires.

Merck, however, has a strong pipeline, remains committed to dividend growth, and yields 3.9%.