Here's why:
* Common Noun: "Antibiotic" is a common noun, referring to a general class of drugs. Common nouns are not capitalized unless they are at the beginning of a sentence or part of a proper noun.
* Not a Proper Noun: Proper nouns are specific names of people, places, or things (e.g., John, London, Mars). "Antibiotic" is not a specific name.
Example:
* Correct: The doctor prescribed an antibiotic for the infection.
* Incorrect: The doctor prescribed an Antibiotic for the infection.