i yageun yakhyoga ppallaseo dahaengiya.

약 means medicine or drug.

약효 means the medicinal effect, the effect of the medicine, or how well the medicine works.

So in this sentence:

• 이 약은 = as for this medicine
• 약효가 빠르다 = its effect is fast / it works quickly

Korean often uses a separate word like 약효 when talking specifically about the medicine’s effect, not just the medicine itself as an object.

It can feel repetitive in English, but it is natural in Korean.

The structure is:

• 이 약은 = as for this medicine
• 약효가 빠르다 = the effect is fast

So the sentence is organized like this:

• Topic: this medicine
• Subject of the description: its effect

A very literal breakdown would be:

• As for this medicine, its effect is fast, so that’s a relief.

Korean often introduces a broad topic first, then gives a more specific subject inside that topic.

This is a very common Korean pattern.

• 은/는 marks the topic
• 이/가 often marks the subject of the descriptive part

Here:

• 이 약은 = as for this medicine
• 약효가 빠르다 = the medicinal effect is fast

So 은 sets up what you’re talking about, and 가 marks what is actually being described as fast.

If you changed the particles, the nuance would change. With 이 약은, the speaker is presenting this medicine as the topic and then commenting on one of its qualities.

Because 빠르다 is an adjective in Korean, and here it is describing 약효.

• dictionary form: 빠르다 = to be fast
• connective form: 빨라서 = because it is fast / since it is fast

So:

• 약효가 빨라서 = because the medicine’s effect is fast

By contrast, 빨리 is an adverb, meaning quickly.

Compare:

• 약효가 빠르다 = the effect is fast
• 약이 빨리 듣다 = the medicine works quickly

Both ideas are possible in Korean, but this sentence uses the adjective pattern, not the adverb pattern.

Yes, it is natural in Korean.

Korean often describes results or effects as being fast or slow:

• 약효가 빠르다 = the medicine takes effect quickly
• 효과가 빠르다 = the effect is quick
• 회복이 빠르다 = recovery is quick

So even if the English wording sounds unusual when translated literally, the Korean expression itself is normal.

A smoother English translation would be something like:

• I’m glad this medicine works quickly.
• It’s a relief that this medicine takes effect quickly.

Here -아서/어서 connects the first part to the second part and gives a reason/cause.

• 약효가 빨라서 = because the medicine works quickly
• 다행이야 = that’s a relief / I’m glad

So together:

• 약효가 빨라서 다행이야 = It’s a relief that the medicine works quickly

This pattern is very common with emotions or judgments:

• 비가 안 와서 다행이야. = I’m glad it’s not raining.
• 늦지 않아서 다행이에요. = It’s a relief that we’re not late.

다행이다 means something like:

• to be fortunate
• to be a relief
• to be lucky

So 다행이야 is the casual form of 다행이다.

In this sentence, it expresses relief:

• 이 약은 약효가 빨라서 다행이야. = It’s a relief that this medicine works quickly. = I’m glad this medicine takes effect quickly.

It does not just mean simple happiness. It usually has the nuance of:

• good thing
• fortunately
• what a relief

Because 다행이야 is an informal/casual ending.

Forms of the same expression:

• 다행이야 = casual
• 다행이에요 = polite
• 다행이다 = plain dictionary-style / written / self-talk tone, depending on context

So the sentence could be changed depending on who you are talking to:

• 이 약은 약효가 빨라서 다행이야.
  casual, to a friend or someone close
• 이 약은 약효가 빨라서 다행이에요.
  polite, more neutral and common in conversation

Yes, but the nuance changes.

• 이 약은 약효가 빨라서 다행이야.
  As for this medicine, its effect is quick, so that’s a relief.
  This sounds like the speaker is presenting this medicine as the topic and commenting on it.

• 이 약이 약효가 빨라서 다행이야.
  This is less natural in most situations, because now 이 약이 sounds more like the grammatical subject, but then 약효가 also appears, which can make the structure feel awkward.

So 이 약은 ... 약효가 ... is the more natural version.

Sometimes yes, but there is a nuance difference.

• 약효 = specifically the medicinal effect
• 효과 = effect/result more generally

So:

• 약효가 빠르다 sounds specifically about medicine
• 효과가 빠르다 can also work, but it is a bit broader and less specifically medical

Since the sentence is about medicine, 약효 is a very fitting word.

A useful pattern here is:

A/V-아서/어서 다행이다
= It’s fortunate that... / I’m glad that... / It’s a relief that...

Examples:

• 시험에 합격해서 다행이야.
  I’m glad I passed the exam.
• 문제가 없어서 다행이에요.
  It’s a relief there’s no problem.
• 약효가 빨라서 다행이야.
  It’s a relief that the medicine works quickly.

This is a very common and useful expression in everyday Korean.