The word "acid" in its name makes boric acid sound somewhat dangerous, but it actually has very low toxicity, minimal impact on the environment and no ability to damage plumbing pipes, so it's safe to put it in your kitchen drain. The question is: Why would you want to do that?
If the answer is to kill insects, such as cockroaches, it probably won't work. If you're trying to bust a clog or deodorize the drain, you might have some success, but there are more effective ways to do both things. There aren't many good reasons for putting boric acid in drains, but if you want to do it, there's little danger of poisoning your family or the environment as long as you don't overdo it.
Perhaps You're Thinking of Borax
Borax and boric acid are both borates, but they aren't the same thing. Borax is better for drain cleaning, but if you're trying to control pests, you should use boric acid.
Borax, or Sodium Tetraborate (Na2B4O7 • 10H2O), is what the 20-mule team transports to markets from the salt flats of the American Southwest, particularly Death Valley. It has been used for centuries as a cleaning agent and is available in the laundry sections of supermarkets. Boric acid, or hydrogen borate (H3BO3), is refined from borax by treating it with an acid. It does whatever borax does, but a little better, and it's also slightly more toxic.
Many of the home remedies that recommend a borate are referring to borax, which can be used to clean laundry, freshen and clean a dishwasher and, yes, even clear a clogged drain. When it comes to pest control, though, boric acid is the go-to product because of its elevated toxicity and smaller granules that are easier for insects to ingest. Because boric acid is more toxic, you generally have to buy it at a drug store.
How Toxic is Boric Acid?
The National Pesticide Information Center identifies boric acid as having low toxicity if you eat it or it contacts your skin. If ingested, it can cause nausea, vomiting and diarrhea, and in high doses, it may cause redness of the skin and rashes. There's only one recorded case of a fatality from boric acid ingestion, and that person died by suicide by ingesting two cups of the powder.
In terms of environmental impact, the boric acid you put in your drain could end up in the groundwater and eventually in the ground itself. However, boric acid exists naturally in the earth's crust and in the soil, so introducing more doesn't create an environmental hazard.
Boric Acid in Drains for Insect Control
Many pest extermination products contain boric acid, but there's a problem with using it in the drain to kill sewer roaches. Boric acid does kill roaches that ingest it, but they usually eat it by grooming themselves after walking through a trail of the powder. To encourage the bugs to do this, the trail must be thin and dry.
If you pour boric acid or borax down the drain for roaches, it will dissolve in the drain water, and there's little chance the bugs will ingest it, although it could happen. However, if you're serious about pest control, it's a better idea to disinfect the drain with bleach, hydrogen peroxide, vinegar or baking soda. Putting bleach down the drain for bugs is also a hit-or-miss approach. Bleach also has more environmental impact than boric acid, but it's more likely to work.