Tuesday was a down day for the stock market, but investors didn't seem particularly concerned about anything in particular. After some big moves higher in recent weeks, market participants seemed mostly just ready for a pause. Large-cap benchmarks like the Dow Jones Industrial Average (^DJI 0.62%), S&P 500 (^GSPC 0.55%), and Nasdaq Composite (^IXIC 0.63%) gave up ground, but the small-cap Russell 2000 Index managed to post modest gains on the day.

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Data source: Yahoo! Finance.

Bucking the downtrend was Amazon.com (AMZN -0.29%), which was up a fraction of a percent on Tuesday. But the news wasn't as good for the competitors that Amazon decided to take on today, and as you'll see, the e-commerce giant is putting itself in position to take on another entire industry with its online model.

Amazon gets into the prescription drug business

Amazon announced its new Amazon Pharmacy service Tuesday. The service will allow customers to buy prescription medications through Amazon's online store, with drugs delivered straight to your home. Users will be able to provide insurance and medical information in a secure profile and then do complete transactions either on a desktop computer or a mobile device using the Amazon app.

Person wearing white lab coat and blue gloves holding pill, with dozens of pills on transparent table.

Image source: Getty Images.

Amazon Pharmacy comes with the same benefits for Prime subscribers that Amazon's other services provide. That includes two-day delivery on pharmacy orders, and that could push more people to subscribe to Amazon Prime as a result.

In addition to its handling of customers who have health insurance plans that cover prescription drugs, Amazon also announced that it will offer a prescription savings benefit through Prime. When paying without insurance, Prime members will be able to save up to 80% off generic medications and 40% off brand-name drugs through what Amazon is dubbing its Prime member Rx savings card. Members will also be able to use the card at 50,000 brick-and-mortar pharmacies across the country.

Carnage across the healthcare industry

Amazon's new service threatens business models of several different players in the healthcare industry, and predictably, those stocks did poorly. Traditional drugstore retail giants were among the hardest hit, as Walgreens Boots Alliance (WBA 0.61%) fell almost 10%, CVS Health (CVS 1.36%) dropped 9%, and Rite Aid (RAD) plunged more than 16%. Drugstore retailers count on bringing customers in with prescription drugs but then selling front-of-store items while they're waiting. Delivery will potentially kill that front-end business, crushing profits.

In addition, the Prime prescription drug discount service is a direct attack against the model that GoodRx Holdings (GDRX -3.98%) uses. Having just come public in September, GoodRx's stock plummeted 23% on Tuesday. That took the stock well below its first-day trading price, and it's now less than 10% above what IPO participants paid.

Watch out for competition

Amazon's latest moves show that many companies are potentially vulnerable to competitive threats, especially as the digital revolution continues to evolve and move forward. Even well-established businesses like CVS and Walgreens aren't immune to potential disruption, and even successful IPOs like GoodRx can find themselves facing existential challenges just months after going public.

Stock market  investors have to be ready for anything. By watching your companies closely, you'll be better prepared to handle setbacks when they come.