Magazine article Drug Topics

Rx Substitution Rates Held Steady in 1991, Survey Shows

Magazine article Drug Topics

Rx Substitution Rates Held Steady in 1991, Survey Shows

Article excerpt

In a year during which there was little new generic encroachment involving major brands, the substitution rate for new brand-name prescription drugs leveled off for the first time in recent years. According to Market Measures Inc.'s 1991 National Substitution Audit, substitution of generics or alternative brands remained at 28%.

Substitution rates remained consistent with 1990 levels in both DAW and two-line prescription form states (30% and 24%, respectively). In addition, substitution in both chain and independent pharmacies held steady at 28%.

The National Substitution Audit also tracks physician prescribing patterns. In 1991, physicians prohibited substitution for 25% of new prescriptions, up from 23% the previous year.

When a brand product loses its patent protection, there is a general pattern to the substitution process. Significant substitution occurs almost immediately following the introduction of generic competition. This is followed by a gradual increase and then a leveling off of substitution.

There is no such clear pattern in physician prescribing or pharmacist dispensing of related co-marketed or multisource products. In the case of co-marketed products such as Merck Sharp & Dohme's Prinivil/Stuart Pharmaceuticals' Zestril and Hoechst-Roussel Pharmaceuticals' DiaBeta/The Upjohn Co.'s Micronase, physicians generally do not prescribe generically. However, loss of patent protection for products such as Schering Corp.'s Proventil/Allen & Hanburys' Ventolin or G. D. Searle & Co.'s Calan/Knoll Pharmaceuticals' Isoptin does not guarantee that physicians will begin prescribing generically. In the former case, only 4% of prescriptions were written generically for albuterol, while in the latter, 36% were written generically for verapamil.

When prescribing one of these equivalent brand-name products, physicians may favor one brand over the other, or they may prescribe both equally. There is no discernible pattern reflected in the prescribing of these co-marketed products, as factors such as manufacturer, price, and sample availability come into play.

Just as physician behavior is hard to predict, pharmacist behavior in dispensing these products can reflect certain tendencies but no predictable trends. In some cases, substitution levels are equal for both brands, while in other instances one brand is substituted twice as frequently as the other.

Generally speaking, however, substitution is relatively low. As with physicians, outside influences such as pricing, stocking, and wholesalers, among others, help determine how the prescription will be dispensed.

Even less predictable than prescribing and dispensing of co-marketed products is the multisource product marketplace. …

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