Questo antibiotico fa effetto dopo poche ore, ma la terapia va finita.

Because antibiotico is a masculine singular noun, so it takes the masculine singular demonstrative questo.

• questo = this (masculine singular)
• questa = this (feminine singular)

So:

• questo antibiotico = this antibiotic

If the noun were feminine, you would use questa.

Fare effetto is a very common Italian expression meaning to have an effect, to work, or to take effect.

So:

• Questo antibiotico fa effetto dopo poche ore
  = This antibiotic works / takes effect after a few hours

It is more idiomatic than translating word-for-word as makes effect, which would sound wrong in English.

You may also hear it with many other things:

• La medicina fa effetto = The medicine is working.
• Il caffè non mi fa effetto = Coffee doesn’t affect me / doesn’t do anything for me.

Because fare effetto is a fixed idiomatic expression. In this phrase, Italian normally does not use an article.

So you say:

• fa effetto = works / has an effect

not usually:

• fa un effetto
• fa l’effetto

Those forms can exist in other contexts, but they mean something more specific, such as produces an effect of a certain kind.

Here, fare effetto is just the standard expression for medicine starting to work.

It means after a few hours.

Breakdown:

• dopo = after
• poche = few
• ore = hours

So the idea is that the antibiotic begins to work within a small number of hours.

Because ore is a feminine plural noun.

• singular: ora = hour
• plural: ore = hours

The adjective poco must agree with the noun:

• poche ore = a few hours
• pochi minuti = a few minutes

So:

• poche matches feminine plural ore
• pochi would be used with masculine plural nouns

In medical Italian, terapia often means treatment or course of treatment, not just therapy in the English psychological or physical-therapy sense.

So in this sentence:

• la terapia va finita

the meaning is closer to:

• the treatment must be completed
• you must finish the course

In the context of antibiotics, this usually refers to finishing the full prescribed treatment, even if you start feeling better.

This is a very important pattern in Italian:

• andare + past participle

It often expresses that something needs to be done, should be done, or must be done.

So:

• la terapia va finita
  literally: the treatment goes finished
  real meaning: the treatment must be finished

This construction is common when talking about instructions, necessity, or what is required.

Other examples:

• Il modulo va firmato. = The form must be signed.
• La porta va chiusa. = The door should be closed.
• Questi esercizi vanno fatti. = These exercises must be done.

Because the past participle agrees with la terapia, which is feminine singular.

• terapia = feminine singular
• therefore: finita = finished (feminine singular)

Compare:

• Il trattamento va finito. = The treatment must be finished.
  (trattamento is masculine singular)

• Le cure vanno finite. = The treatments must be finished.
  (cure is feminine plural)

They are similar, but not always identical in tone.

• la terapia va finita = the treatment must/should be completed
• la terapia deve essere finita = the treatment has to be finished

In this context, both can express necessity, but va finita is very natural and compact in Italian, especially in instructions or general statements.

A more direct version would also be:

• Bisogna finire la terapia. = One must finish the treatment.
• Devi finire la terapia. = You must finish the treatment.

So va finita is not strange or literary here—it is a normal and useful structure.

Ma means but. It shows contrast between the two ideas:

1. Questo antibiotico fa effetto dopo poche ore
   = This antibiotic starts working after a few hours

2. ma la terapia va finita
   = but the treatment must be completed

The contrast is important: even if the medicine starts helping quickly, you still should not stop taking it early.

Yes. Fare effetto is flexible and can be used in different contexts.

For example:

• Questa medicina fa effetto subito. = This medicine works immediately.
• La notizia mi ha fatto effetto. = The news affected me.
• Quel film mi ha fatto un certo effetto. = That film had a certain effect on me / left an impression on me.

In your sentence, though, because of antibiotico, the meaning is clearly medical: it takes effect / it works.

You can break it down word-for-word to understand it, but the best English translation should be more natural.

Word-for-word:

• Questo = this
• antibiotico = antibiotic
• fa effetto = makes/does effect → idiomatically works / takes effect
• dopo poche ore = after a few hours
• ma = but
• la terapia = the treatment / therapy
• va finita = must be finished

Natural English:

• This antibiotic starts working after a few hours, but the treatment must be completed.

or more naturally in everyday medical advice:

• This antibiotic takes effect after a few hours, but you must finish the course.

Yes, it sounds completely natural in a medical context. It is slightly neutral-to-formal, which fits a doctor, pharmacist, leaflet, or health website.

A doctor might also say more conversational versions such as:

• Questo antibiotico fa effetto dopo poche ore, ma devi finire la terapia.
• Anche se ti senti meglio, la terapia va finita.

So the original sentence is natural and standard Italian.