First, antibiotics can lose their effectiveness because the population is exposed to them in different ways; for example, plants and animals treated with antibiotics similar to those used for humans. Some antibiotics used at the wrong times generate the biggest problem: bacterial resistance. The bacterium will survive and the treatment will not be effective, making the antibiotic useless for some treatments. Secondly, many drugs will have side effects, and when a person uses them constantly, it will not only cause acute effects, but also cause chronic effects that can be fatal. The chronic effects are serious, because they can affect the liver and kidneys which are responsible for cleaning the human body, excreting useless or toxic metabolites. Thirdly, there are interactions of antibiotics with other substances that influence the treatment, causing more or less effects. The most common examples are oral contraceptives, which lose their effects, and alcohol, which increases side effects. However, there is no evidence yet to support these findings. However, doctors advise against drinking alcohol and antibiotics together and using alternative contraceptive methods when a woman is on antibiotic therapy. For these reasons the improper use of antibiotics is dangerous; may decrease the effectiveness of treatment, have side effects and interact with others