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I think it's time I devoted a column to a phenomenon known as forced substitution. This is a growing practice that occurs when an insurance carrier tells its patients that it will only cover certain drugs in a class, but not others—either the pill being pushed is generic and therefore cheaper for the insurance company, or else the company has negotiated prices to make up its formulary and has excluded those drugs which it cannot get at bargain prices.

This practice may or may not be good business, but it is definitely not good medicine. Drugs are not identical. Commonly prescribed medicines—such as ace inhibitors or beta blockers—vary slightly from one product to the next. One drug in the class may cause a patient to experience the side effects of dizziness or fatigue or rash, whereas another drug may not.

A dramatic example involves cholesterol-lowering statins. It is often difficult enough to convince a healthy patient to start a statin in the first place to reduce the abstract risk of developing heart disease later on. On multiple occasions, I have been compelled to switch a patient off Lipitor or Crestor because their insurance would only cover the newly generic simvastatin (the generic of Zocor). Simva, as it is known in the trade, may not be quite as potent as the other two, but it is comparable, and is clearly cardio-protective as well. But a patient may well experience muscle problems or elevated liver enzymes on the second drug that he or she didn't experience on the first. And once these side effects occur, a patient may be reluctant to take any statin drug at all, even the one that was so well tolerated initially.

Rather than saving healthcare dollars, ironically, forced substitution may end up costing more in the long run—one must factor in the cost of unnecessary side effects or of a preventable illness like heart disease.

Marc Siegel, MD, is an internist and associate professor of medicine at New York University and the author of False Alarm: The Truth About the Epidemic of Fear

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