1) Imagine a child in school. The child bothers his classmates by talking during class and getting those classmates in trouble. One behavior is identified as creating the problem.
The teacher writes the parents of the child:
I have a concern about your son: He constantly talks out of turn in class.
Please bear in mind, though, that generally, the singular is not as used as the noun concern on its own. Both could be used here.
2) Imagine a child in school. The child talks in class when he shouldn't, throws paper airplanes and runs down corridors shouting.
Here, the school administrator might write:
I have concerns about your son. He does A, B and C, which is disruptive to other students. (they are enumerated).
3) So, concern can be used as a general idea: There is concern about your child's behavior. [this is a general thing]. But, as you see in 1) and 2) they can also be specific.
So, let's look at your idea in the context of drugs (medications).
There is concern in the medical community about these drugs. Doctors fear that these drugs could [etc.]. concern as an uncountable noun is used here as a general principle in the medical community. When an uncountable noun is used as a notion or idea, there is no s. Love is a positive thing.** Concern for others is a social good."
There is a concern in the medical community that these drugs could induce a negative long-term effect regarding X. There is one specific concern; concern is used as a countable noun.
There are concerns in the medical community that these drugs will A, B and C. there are three specific concerns: A, B and C.
There are many nouns in English that can be used either uncountably or countably.
Others include: fear, delight, love, hate, coffee, tea, determination, etc.
So, in summary, ask yourself:
- Am I expressing a general notion? Then, don't use the determiner a or s for concern.
- Am I expressing one specific notion? Use a concern.
- Am I expressing more than one problem? Use concerns
Tea is a wonderful drink. A tea in the morning will wake you up. But yesterday I drank too many teas [read: cups of tea] and therefore did not sleep very well last night.