Is the phrase "harmful symptoms" acceptable (does it sound natural) to a native English speaker? If so could you suggest a sentence using it?
The Shorter OED's main definition of symptom, in the medical sense is:
A physical, or mental phenomenon, circumstance, or change of condition, arising from and accompanying a disorder; and constituting evidence of it; a characteristic sign of a particular disease.
So symptoms are like messengers, indications of what is going on. Though the underlying condition may be harmful, a symptom cannot be harmful, or otherwise.
So though one might speak of "worrying", "disquieting" or even "unconcerning" symptoms - the expression "harmful symptom" is illogical and hence not acceptable. Having said that, the term is undoubtedly in widespread lazy use.
Let's say I have a mild rash. It itches. That is a symptom. But if it itches so much that I scratch it, now I'm causing damage to the skin. Harmful symptom.
Let's say someone has OCD and suffers from intrusive thoughts. Not only is that unpleasant, but this symptom interferes with sleep, so now on top of everything else, the individual is developing a sleep disorder. The more short on sleep this person is, the harder it is to fall and stay asleep. The individual is now in a downward spiral. Here, to, the symptom is harmful.
The term "harmful symptom" is not often used in medicine; the Google search for site:gov "harmful symptom" gives only 5 results, and site:gov "harmful symptoms" 595 results, but in most cases, it is used inappropriately: sometimes instead of a side effect (of drugs), complication, symptom of a severe disease or just as a severe symptom.
Harmful = of a kind likely to be damaging (Merriam-Webster).
A symptom is any subjective evidence of disease (MedTerms), something what a patient feels (pain...) or observes (rash...); it is a "passive" thing that can't really "do" something, like cause harm or damage.
Now, someone could say that cough is a harmful symptom when it prevents someone from sleep; this can be disturbing but not necessary harmful (damaging). You can just say that cough results in lack of sleep.
Loud music is annoying when it prevents someone from sleep and harmful when it results in hearing impairment.
Harm is a hard word.