The substances which stop or slow down the reaction are called .

Explanation

**Step-by-Step Solution:**

1. **Understanding the Reaction Process:**
- In biochemical reactions, enzymes play a crucial role by binding to substrates at their active sites to facilitate the conversion of substrates into products.

2. **Identifying Substances that Affect Reactions:**
- Sometimes, there are substances that can interfere with the normal functioning of enzymes. These substances can either slow down or completely stop the reaction.

...