There are several substances that can be poured down the drain to kill roaches. Here are some of the most effective and environmentally friendly solutions:

1. Boiling water: One of the simplest ways to kill roaches that might be using your drains as an access point is to pour boiling water down the drain. Let warm water run for a few minutes, then follow up with 3-4 cups of boiling water.

2. Borax: The mineral borax (sodium borate) can also kill cockroaches if they ingest it. It is frequently used in cleaning powders, laundry detergents, and cosmetics.

3. Baking soda and vinegar: Start with ¼ cup of baking soda. Pour it down the drain, then follow with ½ cup of vinegar. Pour boiling water down the drain a few minutes later to clear it.

4. Bleach: Bleach can be used to kill roaches hiding in drains. Pour 1/4 cup baking soda down the drain and follow it with 1/2 cup of bleach. Let it sit for an hour, then flush with hot water.

Its important to note that pouring chemicals down the drain isnt always safe for you or your sewer. In addition, some substances may create clogs or add to the pressure in your pipes. Therefore, its best to use environmentally friendly solutions whenever possible.