There are a myriad of reasons why this is bad, and feel free to ask for explanation of any. This is just off the top of my head.

• A single large monopolistic corporation should not have access to your purchasing and medical history
• A single breach can now mean purchasing history and medical history
• Huge potential for ads targeted for a person showing up on a device that isn't strictly theirs (like a family computer) that by doing so endangers their well-being.
• Reselling of "anonymous" medical info (it'll happen) that is super easy to tie back to previously purchased Amazon info.
• Third-parties who purchase info not having as hardened security as Amazon leaving data out in the open
• AI/algoithmic price-gouging on items based on medical data
• Even more god damned lobbying for anti-patient/anti-consumer based changes to health care law with the financial power of Amazon behind it
• This sets the bar for other Big Tech and what they also have to do to stay competitive
• Contacting Amazon customer service for errant prescription/care charges

Do you really want Jeff Bezos and an Amazon Board of Directors death panel deciding what care you receive?