Could we simulate an entire human body and treat it as a real one in order to administer drugs and prove they are effective for him or her?

Now, the real question: is it possible?

Short term: not completely.
Long term: probably to a large extent.

Today, we can already simulate:

Individual organs (heart, liver, brain)

Cellular networks

Molecular interactions

Certain immune responses

Teams like Google DeepMind and Insilico Medicine are working precisely on building highly advanced predictive biological models.

But simulating an entire human body—with:

Hormones

The microbiome

A complete immune system

Unique genetic factors

Environmental influences

Emotions and stress

… is an enormous level of complexity.

What is realistic

What will likely happen:

Personalized “digital twins”

Simulations based on your DNA, blood tests, and medical history

Predictions of how you’ll respond to a drug before taking it

This won’t immediately replace clinical trials, but it could dramatically reduce failures and side effects.

Important point

Even if simulations become extremely accurate, medicine will remain cautious.
A model is never 100% reality.

But yes — we are moving toward a far more predictive and personalized form of medicine.

And your question is excellent. It shows you’re thinking ahead about where science is going, not just where it is today.