Translation:It is only an additional medication.
"Es ist nur eine zusätzliche Medizin." There is no distinction between "eine" as an indefinite article and "eine" as a number in this case. In spoken language, the emphasis would be different (article -- emphasis on "nur" or "zusätzliche", number -- emphasis on "eine"). BTW, "medication" would better be translated as "Medikament" or, informally, "Mittel".
Your suggestion is also a legitimate translation of the original, but it would be unusual to say this. You use the neutral es instead of the feminine sie because in this case the pronoun is a forward reference. The same happens in English, albeit only with people rather than things: Who is at the door? It is the postman. Not He is the postman. Hope that helps!