Drugs that bind to the receptor site and inhibit its natural function are called :

Explanation

To solve the question, we need to identify the type of drugs that bind to receptor sites and inhibit their natural function. Let's break down the solution step by step.

### Step-by-Step Solution:

1. **Understanding Receptor Sites**:
- Receptor sites are specific locations on a cell where drugs or other molecules can bind. This binding can influence the activity of the cell.

2. **Types of Drugs**:
- Drugs that interact with receptor sites can be categorized into two main types: **agonists** and **antagonists**.

3. **Definition of Agonists**:
- Agonists are drugs that bind to receptor sites and activate them, promoting their natural function. They mimic the action of the natural substances that normally bind to these receptors.

4. **Definition of Antagonists**:
- Antagonists are drugs that bind to receptor sites but do not activate them. Instead, they block or inhibit the natural function of the receptor. This means they prevent the natural substances from binding and exerting their effects.

5. **Conclusion**:
- Since the question specifically asks for drugs that bind to receptor sites and inhibit their natural function, the correct answer is **antagonists**.

### Final Answer:
Drugs that bind to the receptor site and inhibit its natural function are called **antagonists**.

---