Vaig a la farmàcia per comprar un medicament.

Yes, that is a very common point of confusion.

Here, vaig is the present tense of anar (to go) for I:

• vaig = I go / I’m going

So:

• Vaig a la farmàcia = I’m going to the pharmacy

But Catalan also uses vaig + infinitive to form a past tense called the periphrastic past:

• vaig comprar = I bought
• vaig menjar = I ate

So the difference is:

• vaig a la farmàcia → vaig is the main verb, meaning I go
• vaig comprar → vaig is helping form a past tense, meaning I bought

In your sentence, because vaig is followed by a la farmàcia and not directly by an infinitive, it means I go / I’m going.

Catalan often drops subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• Vaig already tells you it is I
• So jo is usually unnecessary

You could say:

• Jo vaig a la farmàcia...

But that usually adds emphasis, like:

• I’m the one going to the pharmacy
• As for me, I’m going to the pharmacy

In normal speech, Vaig a la farmàcia... is the natural choice.

Catalan normally uses the preposition a (to) before places you go to, and nouns usually take an article.

So:

• a = to
• la farmàcia = the pharmacy

Together:

• a la farmàcia = to the pharmacy

This is very similar to English to the pharmacy.

Because the article is la, and in Catalan:

• a + el = al
• de + el = del

But with la, there is normally no contraction:

• a + la = a la
• de + la = de la

So:

• Vaig al supermercat = I’m going to the supermarket
• Vaig a la farmàcia = I’m going to the pharmacy

That is why a la stays as two words.

Because the noun farmàcia is grammatically feminine in Catalan, so it takes la.

• la farmàcia = the pharmacy

Grammatical gender in Catalan does not always match natural gender; places and objects are simply assigned a gender. In this case, you just learn it as part of the noun:

• la farmàcia
• una farmàcia

A useful habit is to memorize nouns with their article.

Per comprar means to buy in the sense of in order to buy or for buying.

So:

• Vaig a la farmàcia per comprar un medicament
• I’m going to the pharmacy to buy a medicine / medication

Here per + infinitive expresses purpose:

• Estudio per aprendre = I study to learn
• Truco per preguntar = I’m calling to ask

So per comprar tells you why the person is going to the pharmacy.

In modern Catalan, when you express purpose before an infinitive, per is the normal choice.

So:

• per comprar = to buy / in order to buy

You may sometimes encounter per a in other contexts, but with an infinitive of purpose, per is the standard form learners should use.

So this sentence is perfectly natural:

• Vaig a la farmàcia per comprar un medicament

Un medicament means a medicine / a medication / a drug.

In many contexts, medicament is the more precise word for an actual pharmaceutical product, especially something you buy at a pharmacy.

Medicina can also mean medicine, but it often has broader meanings, such as:

• the field of medicine
• medical treatment
• a remedy

So in a pharmacy context, un medicament is a very natural choice.

Also note the gender:

• un medicament → masculine
• una medicina → feminine

Normally, Catalan needs an article with a countable singular noun when you mean a/an.

So:

• un medicament = a medicine / a medication

If you said just medicament here, it would sound incomplete.

The article un works just like a/an in English.

It is grammatically present tense, but like the English present, it can often express an action happening now or around now.

So Vaig a la farmàcia can mean:

• I go to the pharmacy
  or, more naturally in many contexts,
• I’m going to the pharmacy

Which English translation fits best depends on context. In everyday speech, this sentence often means I’m going to the pharmacy.

This is a very normal Catalan word order:

• Vaig = verb
• a la farmàcia = where you are going
• per comprar un medicament = purpose

So the structure is roughly:

• [Verb] + [destination] + [purpose]

This matches English fairly closely:

• I’m going + to the pharmacy + to buy a medicine

Catalan can sometimes change word order for emphasis, but this version is the most neutral and natural.

A simple approximate pronunciation for an English speaker is:

• VAITCH ah luh fur-mah-SEE-uh per koom-PRAHR oon muh-dee-kuh-MEN

A few notes:

• vaig sounds roughly like batch with a v/b-like beginning, though the exact sound depends on accent
• the stressed syllable in farmàcia is -mà-
• the stressed syllable in medicament is the last one: -ment

If you want a more Catalan-like breakdown:

• vaig
• a la far-MÀ-ci-a
• per com-PRAR
• un me-di-ca-MENT

The accent mark shows the stress:

• farmàcia → stress on mà

Without the accent, a reader might expect a different stress pattern. Written accents in Catalan are very useful because they help show correct pronunciation.

So:

• far-MÀ-ci-a

not

• far-ma-CI-a

In some varieties or informal usage, learners may hear structures with a + infinitive, but for expressing purpose here, per comprar is the standard and safest choice.

So for a learner, the best version is:

• Vaig a la farmàcia per comprar un medicament

If your goal is correct, standard Catalan, use per + infinitive for this kind of in order to meaning.