I, personally, never reuse cooking oils. The foods you cook the oils in will cause the oils to go rancid faster.

A recent study found that a toxin called 4-hydroxy-trans-2-nonenal (HNE) forms when such oils as canola, corn, soybean and sunflower oils are reheated. Consumption of foods containing HNE from cooking oils has been associated with increased risks of cardiovascular disease, stroke, Parkinson'?s disease, Alzheimer's disease, Huntington's disease, various liver disorders, and cancer. Once absorbed in the body, HNE reacts with DNA, RNA and proteins affecting basic cellular processes.

That being said:

Reusing cooking oil has been done for ages. There really isn't a problem, if done properly. The greatest hazard is allowing the fat to become rancid (spoiled) and deteriorated to the point it produces undesirable flavors and odors. Besides ruining what would have been a perfectly good meal, rancid oils also contain free radicals that are potentially carcinogenic. Rancid oil has fewer antioxidants but is not poisonous.

To re-use oil safely, use these tips: ·         Strain it through a few layers of cheesecloth to catch any food particles. Be careful with hot oil, though, because you can easily get burned. ·         Shake off excess batter from food before frying it. ·         Use a good thermometer to fry foods at 190°C. ·         Turn off the heat after you are done cooking. Exposing oil to prolonged heat accelerates rancidity. ·         Don't mix different types of oil. ·         Store oil in a cool, dark place. ·         Avoid iron or copper pots or pans for frying oil that is to be reused. These metals also accelerate rancidity. Signs of Deteriorated Oil: ·         Oil darkens with use because the oil and food molecules burn when subjected to high/prolonged heat. ·         The more you use an oil, the more slowly it will pour. Its viscosity changes because of changes to the oil's molecular structure. ·         Loose absorbent particles accumulate as sediment at the bottom of the storage container or are suspended in the oil. ·         When smoke appears on the oils' surface before the temperature reaches 190 degrees C (375 degrees F), your oil will no longer deep-fry effectively. ·         If the oil has a rancid or "off" smell or if it smells like the foods you've cooked in it, it should be discarded.