Therapeutic drug substitutions have the potential to double or even triple annual cost savings for payers compared with the savings achieved through generic substitutions, say US researchers.

They find that, for a population of enrolees in the US federal Medicare programme's prescription drug benefit (Part D), generic substitutions resulted in annual savings of $127-$160 per beneficiary, while therapeutic substitutions had the potential to reduce annual costs by $389-$452.

"In other words, therapeutic drug substitutions could result in two to three times greater annual cost savings than possible generic substitutions," say the researchers, O Kenrik Duru et al from the University of California. Los Angeles, reporting their findings in the Journal of General Internal Medicine (JGIM).

Therapeutic drug substitutions involve the use of less expensive substitutes that are not equivalent but have a similar treatment effect as the original medication. About 90% of US hospitals regularly make therapeutic substitutions, but it is not yet common practice in outpatient settings.

The study analysed data for 145,056 low-income subsidy Part D benefit beneficiaries and 1,040,030 non-low-income subsidy enrolees, nearly half of whom were eligible for generic and/or therapeutic substitutions. 

For each generic substitution for low-income subsidy patients, they found that the government would save an average of $156 per year, while for each therapeutic substitution, it would save $126 and the health plan would save $305 per year.

Patients not receiving the low-income subsidiary would receive $138 a year for each generic substitution, while for each therapeutic substitution, they would save an average $113 per year and the annual saving to the health plan would be $276.

Medicare Part D provides drug coverage for almost 28 million Americans, who each take an average of five medications and fill more than 30 prescriptions each year. Although the burden of rising out-of-pocket drug costs is likely to ease as a result of changes to Part D coverage, additional strategies to reduce drug costs, including out-of-pocket, health plan and government subsidy costs, are still very much needed,  say the researchers.

"While drug costs differ across health systems and vary over time, these findings indicate the importance of examining generic and therapeutic substitutions as a next step to lowering drug costs within Medicare," they write.

"Ultimately, however, both physicians and patients will need to make informed decisions about the various tradeoffs associated with those substitutions," they add.