kono kusuri ni ha kouka ga aru.

には is に + は.

• に marks the thing something applies to, like for / in / with regard to
• は marks that whole phrase as the topic or adds contrast/emphasis

So この薬には効果がある means something like:

• As for this medicine, it has an effect
• This medicine does have an effect
• There is an effect in/with this medicine

In natural English, we usually just say This medicine is effective or This medicine works.

Using には often gives a slight sense of with this medicine or at least in this medicine / as for this medicine.

In 効果がある, 効果 is the thing that exists.

The basic pattern is:

• X が ある = there is X / X exists

So:

• 効果がある = there is an effect

Here, ある is not just to exist in a physical sense. It often means to be present, to be possessed, or to occur.

If you said 効果はある, that would put effect itself in the topic/contrast position, as in:

• There is an effect, at least
• As for the effect, it exists

That is possible in the right context, but 効果がある is the normal neutral phrasing.

に has several uses. Here it does not mean movement to a place.

In this sentence, に marks the thing in which, for which, or with regard to which the effect exists.

So:

• この薬に効果がある = This medicine has an effect / There is an effect in this medicine

This use of に often appears with ある when talking about something being present in or associated with something.

It is a bit different from literal location, but related.

You can say この薬は効果がある, and it is understandable. But この薬には効果がある is more natural and idiomatic.

The difference is subtle:

• この薬は効果がある
  = This medicine is effective
  More direct, but slightly less standard in this exact pattern.

• この薬には効果がある
  = This medicine has an effect / This medicine is effective
  Feels more like with this medicine, there is an effect

Also, には can suggest contrast:

• この薬には効果があるが、あの薬にはない。
  This medicine is effective, but that one isn’t.

So には is very common here.

Normally, learners first meet ある as to exist for non-living things:

• 本がある = There is a book

But ある also has broader meanings such as:

• to have
• to exist
• to be present
• to occur

So in 効果がある, it means:

• there is an effect
• an effect is present
• it has effectiveness

That is why the whole sentence can be translated naturally as:

• This medicine is effective
• This medicine works

Yes. この薬は効く is a very common and natural sentence.

It means:

• This medicine works
• This medicine is effective

The nuance is a little different:

• この薬には効果がある sounds a bit more formal or objective
• この薬は効く sounds more direct and conversational

So both are correct, but they are used in slightly different styles.

Because この薬の効果 means the effect of this medicine, which is a noun phrase.

For example:

• この薬の効果は高い。
  The effect of this medicine is strong.

But in your sentence, the meaning is not the medicine’s effect exists as a possessed noun phrase. Instead, it is saying that there is effectiveness in this medicine.

That is why Japanese uses:

• この薬には効果がある

not:

• この薬の効果がある

The latter sounds unnatural in this context.

The sentence is read:

• この くすり には こうか が ある。

Word by word:

• この = this
• 薬（くすり） = medicine
• 効果（こうか） = effect / effectiveness
• ある = to exist / to have

So the full reading is:

• このくすりにはこうかがある。

この means this, and it must be followed by a noun:

• この薬 = this medicine

It refers to a medicine close to the speaker, or one currently being discussed.

Compare:

• この薬 = this medicine
• その薬 = that medicine (near the listener / previously mentioned)
• あの薬 = that medicine over there

So この is just identifying which medicine we are talking about.

It is grammatically plain, but the wording is somewhat neutral or slightly formal because of 効果がある.

• この薬には効果がある。
  plain style, neutral/objective
• この薬には効果があります。
  polite style

In everyday speech, people may more often say:

• この薬、効くよ。
• この薬は効きます。

So the sentence you have is natural, but it sounds a little more like explanatory, written, or factual language.

Yes, especially because of には.

は often creates a contrastive feeling, even if it is subtle. So この薬には効果がある may suggest something like:

• This medicine, at least, is effective
• This medicine does have an effect

That contrast may or may not be important depending on context.

For example:

• この薬には効果があるけど、あの薬には効果がない。
  This medicine is effective, but that medicine is not.

Without context, the sentence can still be understood simply as a neutral statement.

Using the same pattern:

Negative

• この薬には効果がない。
  This medicine has no effect. / This medicine is not effective.

Past

• この薬には効果があった。
  This medicine was effective. / This medicine had an effect.

Polite negative

• この薬には効果がありません。

Polite past

• この薬には効果がありました。

The core expression stays the same:

• 効果がある
• 効果がない
• 効果があった

No. Japanese is normally written without spaces.

So the normal written form is:

• この薬には効果がある。

Spaces are often added in teaching materials to help beginners see the parts more clearly:

• この　薬　に　は　効果　が　ある。

That spacing is just for learning. In real Japanese writing, you usually will not see it.