Tôi bị dị ứng.

Vietnamese often does not use a separate verb like English to be in sentences about states or conditions.

So Tôi bị dị ứng already works as a complete sentence:

• Tôi = I
• bị dị ứng = have an allergy / be affected by an allergic reaction

You should not add là here.
Tôi là dị ứng is incorrect.

Bị often shows that something unpleasant is happening to someone, or that someone is affected by a problem.

In health-related sentences, bị often means something like:

• have
• suffer from
• be affected by

So Tôi bị dị ứng means I have an allergy or I’m having an allergic reaction.

It is not exactly the same as the English passive, even though it sometimes feels a little similar.

Dị ứng means allergy or allergic reaction, and in many sentences it can also function like to be allergic.

That means the same expression can work in different ways depending on context:

• dị ứng thuốc = drug allergy
• Tôi dị ứng với tôm = I’m allergic to shrimp

So it is best to learn dị ứng as one unit rather than trying to translate each part separately.

Yes, it is natural, especially if the context is already clear.

For example, if you are talking to a doctor and suddenly have symptoms, Tôi bị dị ứng can mean:

• I’m having an allergic reaction
• I have allergies

But in real conversation, people often add more information, such as the cause or symptoms:

• Tôi bị dị ứng hải sản. = I’m allergic to seafood.
• Tôi đang bị dị ứng, nổi mẩn đỏ. = I’m having an allergic reaction and breaking out in a rash.

If you only say Tôi bị dị ứng, someone may ask Dị ứng gì? = Allergic to what?

A very useful pattern is:

Tôi dị ứng với + noun

Examples:

• Tôi dị ứng với đậu phộng. = I’m allergic to peanuts.
• Tôi dị ứng với phấn hoa. = I’m allergic to pollen.

You can also hear:

• Tôi bị dị ứng với thuốc này. = I’m allergic to this medicine.

In everyday Vietnamese, people sometimes omit với, especially with common categories:

• Tôi bị dị ứng hải sản.
• Tôi dị ứng thuốc penicillin.

But for learners, dị ứng với + noun is a very safe pattern.

They overlap, but the feeling is a little different.

• Tôi bị dị ứng often focuses on the fact that you are being affected by the allergy, sometimes as a current problem.
• Tôi dị ứng với... more directly states what you are allergic to.

Compare:

• Tôi bị dị ứng. = I’m having allergy problems / I’m having an allergic reaction.
• Tôi dị ứng với tôm. = I’m allergic to shrimp.

Both patterns are common.

It can mean either one, depending on context.

If you want to make right now very clear, say:

• Tôi đang bị dị ứng. = I’m currently having an allergic reaction.

If you want to state a general fact, it is often clearer to name the trigger:

• Tôi dị ứng với mèo. = I’m allergic to cats.

So context does a lot of work here.

Sometimes, but bị is more natural in many everyday situations.

Tôi có dị ứng by itself does not sound as natural as Tôi bị dị ứng or Tôi dị ứng với...

However, có can appear in more formal or medical-style statements, such as:

• Tôi có dị ứng với penicillin. = I have an allergy to penicillin.
• Tôi có tiền sử dị ứng. = I have a history of allergies.

So có is possible, but it is not the most basic or common pattern for this exact sentence.

A rough guide is:

• tôi: similar to toy, with a level tone
• bị: roughly like bee, but with a low, heavy tone
• dị: roughly like zee in the North or yee in the South, also with a low, heavy tone
• ứng: roughly like ung with a rising tone; the vowel ư does not have an exact English equivalent

The trickiest parts for English speakers are usually:

• the tones on bị, dị, and ứng
• the vowel ư in ứng

Not always. Tôi is a neutral and safe choice, especially in formal situations or with strangers.

But Vietnamese pronouns depend a lot on who you are talking to. For example:

• a younger person speaking to a doctor might say Em bị dị ứng.
• a child might say Cháu bị dị ứng.

So the structure of the sentence stays the same, but the word for I may change depending on age and relationship.

Yes. Besides the medical meaning, dị ứng với... can also mean to really dislike or not be able to stand something.

For example:

• Tôi dị ứng với mùi thuốc lá. = I’m allergic to cigarette smoke.
• Tôi dị ứng với kiểu nói đó. = I can’t stand that way of speaking.

In Tôi bị dị ứng, though, the normal interpretation is the literal medical one.