Drug purchases are evaluated according to available offers from drug manufacturers, based on the equivalent dosage regimens for similar drugs. Equivalent doses among drugs are calculated. Purchasing amounts of equivalent doses of drugs are reviewed. The costs of the equivalent doses of the drugs are compared, taking into account contractual price adjustments, such as discounts and rebates. Subsequent purchasing amounts of the drugs are determined, using results from the comparisons. The new purchasing amounts of the equivalent doses of the drugs are produced.