Imam alergiju na ovaj lek.

In Serbian, the verb ending already shows the subject, so imam by itself means I have.

• imam = I have
• imaš = you have
• ima = he/she/it has

Because of that, ja is often omitted unless you want emphasis or contrast.

• Imam alergiju na ovaj lek. = neutral, normal
• Ja imam alergiju na ovaj lek. = I am the one who has the allergy

Because alergiju is the accusative singular form of alergija.

The verb imati (to have) normally takes a direct object in the accusative case:

• alergija = nominative, dictionary form
• alergiju = accusative singular

So:

• Imam alergiju. = I have an allergy.

This is very common in Serbian:

• Imam knjigu. = I have a book.
• Imam sestru. = I have a sister.

With alergija, Serbian usually uses na to express what someone is allergic to.

So:

• alergija na lek = allergy to a medicine
• alergija na polen = allergy to pollen
• alergija na mačke = allergy to cats

Even though na often means on or onto, here it corresponds to English to in the phrase allergy to. This is just the normal Serbian pattern.

Ovaj lek is the correct form because lek is a masculine singular inanimate noun, and after na here it is in the accusative.

Important detail: for masculine inanimate nouns, the accusative is the same as the nominative.

So:

• nominative: ovaj lek
• accusative: ovaj lek

That is why the phrase stays ovaj lek.

Compare with a masculine animate noun, where the accusative changes:

• nominative: ovaj čovek = this man
• accusative: ovog čoveka = this man

Ovaj means this.

It agrees with the noun in gender, number, and case. Since lek is masculine singular, the matching form is ovaj.

Some related forms are:

• ovaj lek = this medicine
• ova knjiga = this book
• ovo dete = this child

So ovaj is not a fixed word with only one form; it changes depending on the noun it describes.

No. Lek is the normal Serbian word for medicine or drug/remedy.

A very close variant is lijek, which is used in ijekavian varieties. So you may see:

• lek in much of Serbia
• lijek in Bosnia, Montenegro, and Croatia

In standard Serbian, both ekavian and ijekavian pronunciations exist, so both can be correct depending on the variety being used.

Yes. That is also a very natural sentence.

Compare:

• Imam alergiju na ovaj lek. = I have an allergy to this medicine.
• Alergičan sam na ovaj lek. = I am allergic to this medicine.

Both are correct. The second one uses the adjective alergičan (allergic).

Forms:

• alergičan sam = masculine speaker
• alergična sam = feminine speaker

So if a woman says it, she would say:

• Alergična sam na ovaj lek.

The sentence as given is the most neutral word order:

• Imam alergiju na ovaj lek.

Serbian word order is fairly flexible, so other versions are possible for emphasis, but they may sound more marked:

• Na ovaj lek imam alergiju. = emphasis on this medicine
• Alergiju imam na ovaj lek. = unusual, strong focus

For learners, the safest and most natural version is the original one.

A helpful rough English-style guide is:

• a-lehr-GEE-yoo

A few notes:

• stress is on -gi-
• j in Serbian sounds like English y
• so -giju sounds like -gee-yoo

Also:

• lek sounds roughly like lek with a short e
• ovaj sounds roughly like OH-vai or OH-vy

They would probably understand you, but it is not the standard way to express this meaning.

The normal pattern is:

• alergija na + accusative

So the standard phrasing is:

• Imam alergiju na ovaj lek.

Using od would sound less natural here.

It is in the accusative, because na in this meaning takes the accusative.

So the structure is:

• na
  • accusative

With lek, the accusative singular looks the same as the nominative because it is masculine inanimate:

• nominative: lek
• accusative: lek

But with another noun, you may see a visible difference. For example:

• na ovu knjigu = to/on this book
• na ovog psa = to/on this dog

So even though lek does not visibly change, it is still accusative here.

Yes, depending on what you mean.

• ovaj lek = this medicine
• taj lek = that medicine
• onaj lek = that medicine over there / that one

So you can say:

• Imam alergiju na taj lek.
• Imam alergiju na onaj lek.

The grammar stays the same; only the demonstrative changes.

Usually in this meaning, Serbian prefers the singular:

• Imam alergiju na ovaj lek.

That means I have an allergy to this medicine.

You can use the plural alergije in other contexts, for example when talking about multiple allergies in general:

• Imam alergije na polen, prašinu i neke lekove. = I have allergies to pollen, dust, and some medicines.

So in the original sentence, singular is the natural choice.