When is it okay to use "harmful symptoms"?

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.