I think you could say that you have researched the product and this is what you have found, but let them know that you have not tried the product. You could also let people know about the results for patients ( as the patient remains anonymous).

Adding to what GeoffreyC1 said below, in my opinion there will some items where you have opinions and some where you don't. So it will vary on your site.

You can approach reviews from a secondary research perspective. Meaning that you can get info from article/ book/ website/ authority sites/ google etc. research and come up with you own conclusions.

All the best
Hugh

You state the facts from your research. You don't need to give a personal opinion.

Yes you can write reviews on products you have not tried. But you will need to do research and read others reviews to determine the pros and cons.

It is impossible for affiliate marketers to buy all the products they review by means of a site post. Fortunately with the internet search these days it is possible to find out a lot about a product we never try. YouTube is another source of good information.