At first, availability of drugs would be greatly increased. There is not a single product we consume that we wouldn't consume more of if it were for free.
Drugs are no exception.
Knowing that the federal government would pick up the bill, doctors and patients would have an irresistible incentive to make use of more drugs, creating a boom in demand. As soon as the government realized that the prescription bills were coming in faster than cold water into the Titanic, it would conclude, without much effort, that drugs are way too expensive and that doctors must be over-prescribing them. Bureaucrats from Washington would start pressing the drug companies to cut prices. A government monopoly on the buying end of the prescription drugs' market, added to its present marketing control of all drugs in the US, and to the power it already has to unleash antitrust actions, would give it enough power to force the drug companies to accept its terms. To drug makers, lower prices would mean shrinking margins and even losses.