Combinations of antiretrovirals create multiple obstacles to HIV replication to keep the number of offspring low and reduce the possibility of a superior mutation. If a mutation that conveys resistance to one of the drugs being taken arises, the other drugs continue to suppress reproduction of that mutation. With rare exceptions, no individual antiretroviral drug has been demonstrated to suppress an HIV infection for long; these agents must be taken in combinations in order to have a lasting effect. As a result, the standard of care is to use combinations of antiretroviral drugs.  Combinations usually consist of three drugs from at least two different classes.  This three drug combination is commonly known as a triple cocktail.  Combinations of antiretrovirals are subject to positive and negative synergies , which limits the number of useful combinations.