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Narrow Therapeutic Index Drugs: An Area of Controversy

Some medications have what is called a "narrow therapeutic index" (NTI). NTI means that there is a very small difference between a dose that is ineffective and a dose that is toxic. In other words, there is a specific level of medication in the body that is safe and effective. Anything below that is not effective. Anything above that is not safe, or is toxic.
The "blood-thinner" warfarin (Coumadin®) is a classic example of a medication with an NTI. Other examples of NTI medications include but are not limited to:
Common brand names are given in parentheses, but many of these medications have more than one brand name.
The dose of an NTI medication needed to reach a safe and effective level is different for every person. Let's use warfarin as an example. Patient A is taking 5 mg of warfarin every day, and patient B is taking 10 mg of warfarin every day, but they both have a safe and effective level of warfarin in their body. If you switched their doses, patient A would have a toxic level of warfarin and patient B would have an ineffective level of warfarin.
Even a small change in dose can make a drastic difference in the level of medication in a person's body. Let's use patient A as an example. Remember, patient A was taking 5 mg of warfarin, and the level of medication in their body was just right. We said that if patient A took 10 mg of warfarin, they would have a toxic level of medication. Well, even if their dose of warfarin was increased just to 6 mg, patient A could still get a toxic level of medication in their body. Likewise, if their dose of warfarin was decreased to 4 mg, patient A could get a level of medication that is not effective.
There are many reasons why different people need different doses of NTI medications. Some of these medications are affected by the types of foods we eat or if we take them with food. Some are even affected by how much fluid we drink. Others depend on metabolism (how fast we break down medications), and some people metabolize medications quickly while others metabolize them slowly. The dose of the medication might depend on how much the person weighs. Other drugs can also affect the dose of NTI medications.
Many factors determine the dose of medication that a person needs, and some of these factors change daily, throughout the year, etc. Therefore, the level of medication in a person's body needs to be checked with a blood test every so often to ensure that the level does not get too high or too low. The frequency with which a person needs a blood test depends on which medication they are taking and the level that was in their body when they last had their blood drawn.
The differences between brand-name and generic medications, or even between two different generic medications, are usually small and insignificant. There is controversy over whether or not these differences among products may be significant when it comes to NTI medications, as only a small change in dose could change the level of medication in the body. This is not an argument against generic medications. It is simply a question of safety when switching from one product to another. In reality, the answer to this controversy likely depends on the person.
The FDA doesn't have any special rules when it comes NTI medications and switching from one product to another. Some states, however, have passed laws preventing the pharmacist from switching products (say, from one generic to another) without informing or getting authorization from the prescriber. If possible, it is generally recommended to keep the person on the same product and not switch from one generic to another. If changing products is necessary, it is best to monitor the person and the concentration of medication in their body closely after the switch.
Written by/reviewed by:
Last reviewed by: Kristi Monson, PharmD;
Last updated/reviewed:
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