"Take this medicine three times a day."
Translation:이 약은 하루에 세번 복용하세요.
both OK. whatever, 은/는 doesn't specify how the preceding noun works in the sentence. But 은/는 makes delicate differences.
e.g. (*: weird)
A "그 학교는 정말 커" : #'That school'# is so big.
A* "그 학교가 정말 커"
A: 어제 나는 학교를 갔다. : Yesterday I went to school (merely)
A: 어제 나는 학교는 갔다. : Yesterday I went to 'school'
B: 그럼 잤니? : Then did you sleep?
C: 도서관은 안 가고? : Didn't you go to library?
A: 어제 방이 깨끗했다. : Yesterday the room was neat. (merely)
A: 어제는 방이 깨끗했다. : 'Yesterday' the room was neat.
B: 오늘은 안 그래? : Not today?
A: 어제 방은 깨끗했다. : Yesterday 'the room' was neat.
B: 화장실은 더러웠니? : Was the toilet dirty?
Maybe you got the point. 은/는 can make comparative effect. This particle can be interpreted in various meanings by situation. Written above is just one of them.
복용하다 - to have/take/use/administer the medication.
The notion of a medication is therefore already included in the use of this verb. To use the object particle "을" to mark "이 약" though not wrong, would seem redundant.*
The topic particle "은" on the other hand would help to identify/highlight the drug in question, "이 약" (this drug).
- If the verb "먹다" is used instead, then the object particle would need to be used because 먹다 simply mean to take/consume orally.
이 약을 먹어주세요 - take this drug/medicine etc.
Similar to the convention used for date and time, there is a standard practice when it comes to expressing frequency, repetitive distribution.
[KOR] (cycle time)–에 + Number + (Counter)–씩 =
[ENG] Number + (Counter) per (cycle time)
–씩 is a frequency marker describing the verb as a repetitive action. It is often omitted in speech language.