Prescription drug wholesalers, drug store chains and pharmacy benefit managers generally have seen their stock prices tumble this year, even as the overall S&P 500 healthcare sector has gained 6 percent.

Uncertainty over potential actions to reduce prescription drug costs and competitive threats have taken a toll this year on shares of companies in the U.S. pharmaceutical supply chain. They get their chance to change investor sentiment with quarterly earnings reports in the coming days.

Six S&P 500 stocks – CVS Health, Walgreens Boots Alliance, AmerisourceBergen, McKesson, Cardinal Health and Express Scripts Holding  – collectively recently traded a third below their average valuation of the past five years, based on price-to-earnings estimates, according to Thomson Reuters Datastream.

Fears the U.S. government will take actions to rein in prescription drug prices, an issue trumpeted by U.S. President Donald Trump, have cast a cloud over the stocks.

Just last week, the shares were rattled as the Trump administration proposed a rule to scale back protections that allow rebates between drug manufacturers and insurers and pharmacy benefits managers, among the industry’s so-called middlemen.

Amazon’s designs on health care, in particular its planned acquisition of online pharmacy PillPack, also are pressuring the stocks, as are moves by drugmakers such as Pfizer Inc and Merck & Co to cut some drug prices or roll back increases.

McKesson, which reports results on Thursday with other companies following in next two weeks, has had its shares fall about 15 percent this year. Shares of rival drug wholesalers, Amerisource and Cardinal, have fallen nearly 9 percent and 20 percent, respectively. Drug store chains CVS and Walgreens have dropped about 9.5 percent each. CVS is also a large pharmacy benefit manager (PBM).

Express Scripts shares have climbed more than 6 percent this year. The PBM’s stock price has benefited from the company’s March agreement to be bought by insurer Cigna Corp (CI.N). Even so, the stock recently traded at only 8.3 times earnings estimates for next 12 months, a discount to its five-year average of 12 times.