One thing is that the pharmaceutical companies are raising prices right now due to the threat of price controls by congress.
they are trying to get as much as they can before patents run out and to recover the cost of research that failed.
The biggest increase right now is actually the fault of Valeant, a Canadian company that is buying up other pharmaceutical companies, getting rid of their Research and development departments ( why keep another companies expensive R&D when you can use your own ) and then raising the price on the medications.
the insurance companies can negotiate lower prices because they are considered to purchase in large quantities, most pharmaceutical companies also have lower rates for those that are poor.
the ones that really get hurt are those that dont have a prescription plan and have to pay out of pocket.
Regardless of this, the increase is on average around 19% it has not doubled, although there are a few medications that have doubled.