मुझे दवा से एलर्जी है।

Because Hindi often expresses physical states, feelings, needs, and certain conditions with an oblique/dative-style subject rather than a plain subject.

So instead of saying something structured like I am allergic..., Hindi says something closer to:

To me, there is an allergy from medicine.

That is why मुझे is used, not मैं.

A few similar patterns:

• मुझे भूख है = I am hungry
• मुझे ठंड लग रही है = I feel cold
• मुझे डर लगता है = I feel afraid

So मुझे दवा से एलर्जी है follows a very natural Hindi pattern.

मुझे is the oblique form of मैं used with certain postpositions and constructions. In this sentence, it means something like to me.

So:

• मैं = I
• मुझे = to me / me

In many everyday sentences, मुझे is used where English would simply use I:

• मुझे पता है = I know
• मुझे खुशी है = I am happy
• मुझे दवा से एलर्जी है = I am allergic to medicine

Yes, से often means from, with, or by, but it has several uses. In this sentence, it marks the thing that causes the reaction.

So दवा से एलर्जी means:

• allergy from medicine
• allergic reaction to medicine

English says allergic to, but Hindi commonly uses से in this kind of meaning.

Other examples of से showing source/cause/association:

• मुझे धूल से एलर्जी है = I am allergic to dust
• उसे दूध से परेशानी है = Milk causes him trouble
• मुझे ठंड से खाँसी हो जाती है = I get a cough from cold weather

Because है agrees with एलर्जी, not with मैं.

This is important: in this sentence, एलर्जी is the thing being said to exist, and मुझे is not the grammatical subject in the same way I is in English.

So the structure is more like:

• मुझे ... एलर्जी है = There is an allergy to me...

Since एलर्जी is singular, the sentence uses है.

Compare:

• मैं शिक्षक हूँ = I am a teacher
  Here मैं is the subject, so हूँ is used.
• मुझे बुखार है = I have a fever
  Here Hindi uses the possession/state pattern, so है is used.

A fairly literal breakdown is:

• मुझे = to me
• दवा से = from medicine / to medicine
• एलर्जी = allergy
• है = is

So literally: To me, there is an allergy from medicine.

Natural English: I am allergic to medicine.

This kind of literal-vs-natural difference is very common when learning Hindi.

It is a borrowed word, ultimately from English allergy, but it is very commonly used in Hindi.

Many modern health-related terms in Hindi are borrowed or adapted from English. So एलर्जी sounds completely normal in everyday speech.

You may also hear more Sanskritized or formal medical vocabulary in very formal contexts, but एलर्जी is the usual everyday word.

It can mean:

• medicine in a general sense
• a medicine
• medication

In this sentence, दवा is usually understood in a general or contextual sense:

• I am allergic to medicine
• I am allergic to this medicine if the context makes that clear

If you want to be more specific, you could say:

• मुझे इस दवा से एलर्जी है। = I am allergic to this medicine.
• मुझे कुछ दवाओं से एलर्जी है। = I am allergic to some medicines.

Yes, you may hear मुझे दवा की एलर्जी है, and it can sound natural in some contexts, but मुझे दवा से एलर्जी है is the more direct and common way to say I am allergic to medicine.

The difference is roughly this:

• दवा से एलर्जी focuses on the thing causing the allergy: allergic to medicine
• दवा की एलर्जी can sound more like an allergy related to medicine

In everyday speech, if your goal is simply to say you react badly to medicine, मुझे दवा से एलर्जी है is the safest and most standard choice.

The neutral word order is:

मुझे दवा से एलर्जी है।

Hindi word order is fairly flexible, but this version is the most natural basic form.

You may also hear variations for emphasis, such as:

• दवा से मुझे एलर्जी है।
  This emphasizes medicine
• एलर्जी मुझे दवा से है।
  Grammatically possible, but less neutral and more marked

For learners, the best default is: मुझे + thing + से + एलर्जी है

Examples:

• मुझे धूल से एलर्जी है।
• मुझे पराग से एलर्जी है।
• मुझे मूँगफली से एलर्जी है।

Yes. Hindi often uses the same sentence for both:

• I am allergic to medicine
• I have an allergy to medicine

So मुझे दवा से एलर्जी है can match either English version depending on context.

Hindi does not need to separate those two ideas the way English sometimes does.

You just replace दवा with the specific item:

• मुझे इस दवा से एलर्जी है। = I am allergic to this medicine.
• मुझे उस दवा से एलर्जी है। = I am allergic to that medicine.
• मुझे पेनिसिलिन से एलर्जी है। = I am allergic to penicillin.

This pattern is very productive: मुझे X से एलर्जी है

For example:

• मुझे धूल से एलर्जी है। = I am allergic to dust.
• मुझे दूध से एलर्जी है। = I am allergic to milk.
• मुझे मूँगफली से एलर्जी है। = I am allergic to peanuts.

मुझे दवा से एलर्जी है। is already polite and completely appropriate.

If you want to sound a little fuller or clearer, you could say:

• मुझे इस दवा से एलर्जी है। = I am allergic to this medicine.
• मुझे कुछ दवाओं से एलर्जी है। = I am allergic to some medicines.
• मुझे पेनिसिलिन से एलर्जी है। = I am allergic to penicillin.

In a hospital or pharmacy, being specific is especially helpful.