Before approving a drug for use in the United States, the Food and Drug Administration tests it to determine its risks and benefits. The company that created the drug must submit data about the tests it has conducted in the laboratory and in people to prove that the drug has a real health benefit. If the FDA determines that the drug does, in fact, have a health benefit, they will then weigh that benefit with any possible risks or side effects of the drug.
The FDA compares the frequency and seriousness of any side effects to the seriousness of the disease being treated. Generally, very serious and rare diseases, like cancer, are permitted to be treated with drugs that have more risky side effects. If it is indeed determined that the benefits are shown to outweigh the risks in proportion with the seriousness of the condition, then the FDA will allow the drug to be sold/used in the U.S.