Final answer:
The wrong word in the sentence is 'averse'; it should be corrected to 'adverse' to convey that a harmless substance can sometimes negatively affect a patient.
Step-by-step explanation:
The incorrect word in the sentence should be 'averse' which is a common misspelling. The correct word is 'adverse'. Thus the corrected sentence would be: 'Sometimes a completely harmless substance can have an adverse effect on a patient simply because he or she is worried about experiencing a negative reaction.'
Words that sound similar but have different meanings, like 'averse' and 'adverse', can lead to mistakes that not only confuse the reader but also change the meaning of a sentence. Misspellings can hamper effective communication, which is why it is crucial to understand the context in which words are used and to verify their meanings and spellings when uncertain.