Other than using a rat trap, the only effective method for killing rats is to use a toxic bait, according to the University of California Integrated Pest Management Program. Due to changes in the law in 2011, only disposable, ready-to-use bait stations are available to consumers.
While bait stations can be effective, more serious rat problems usually require a professional pest control expert who is licensed to use other forms of toxic bait or rodenticides. These toxic rat baits use food or some other attractant mixed with a chemical toxin. The most common types of rat toxins are slow-acting anticoagulants, which prevent the blood from clotting. After ingesting the bait over a period of a few days, the rat eventually begins bleeding internally and dies. While these toxic baits can be effective, the San Diego Country Department of Environmental Health recommends that these products only be used outdoors.
For rats that are inside a building, only traps should be used as the toxic bait could cause the rat to die inside the house. This could then lead to other problems, such as unpleasant smells or flies. It is also important to make sure the building is completely sealed up before using poison outside as the rats could find their way inside before they die.