More specifically I want to mention the drug Dexedrine, however, if that can lead to legal problems, I could simply describe it as an Amphetamine prescription.
This is not legal advice. That costs money and this is free.
The fair use of a Trademark in expressive works is a legal topic that has made it to the Ninth Circuit -- one step below the Supreme Court.
Under the Rogers test, the first inquiry is whether the use of the third-party mark has “some artistic relevance”. The threshold for this test is extremely low; basically, if the level of artistic relevance is more than zero, this is satisfactory. If there is greater than zero artistic relevance in the use of the third-party mark, the next analysis is whether the use of the third-party mark explicitly misleads as to the source or content of the work.
To this seems clear, if you use a trademarked product name in your story, and the use isn't intrinsic to the story, then you'll fail the Roger's Test and lose the suit in court and have to pay Billions of $$$$ in damages. Furthermore, if your use is obviously intrinsic to the story, but you damage their brand by your representation of their product, you'll also lose and have to pay Billions of $$$$ in damages.
Alright, so you might not have to pay Billions of $$$$ in damages, you might just have to remove the trademarked product from your work.
More than likely, people might to decline to publish your work since they don' want to hire a battalion of Lawyers to represent them in an expensive lawsuit because your sense of artistic integrity demanded you say the John took two capsules of Dexedrine rather than John popped a couple of Bennies or John downed some uppers (or whatever the street name for Dexedrine is.)
There has been a recent modification of the Roger's test, but it doesn't apply to your question. For it to apply the connection between the trademark product name and the company needs to be effectively synonymous -- Coca-Cola for instance.