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What Is the Therapeutic Index?

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  • Written By: S. Berger
  • Edited By: Shereen Skola
  • Last Modified Date: 20 November 2016
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Most medications have beneficial effects when taken in measured dosages, but can have unwanted, or even harmful effects in larger amounts. It can, therefore, be useful to know the ratio of the average dosage of a drug needed to have an intended therapeutic effect versus the average dosage that can cause harm. This ratio is called the therapeutic index, or the therapeutic ratio.

Generally, the dosages used for calculating the therapeutic index are determined by scientific studies. They are known as the TD50 and the ED50. The TD50 is amount that is toxic to 50% of the individuals in a study, and the ED50 refers to the minimum dose that produces the desired effect in 50% of those studied. From here, the therapeutic ratio is given as the TD50 divided by the ED50.

When animal studies are used to calculate the therapeutic index, the process is slightly different. In this case, the dose that causes 50 percent of the animals in the study to die, or the LD50, is used for the equation. The therapeutic ratio is then found by dividing the LD50 by the ED50.

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Studies that are used to find the effective dose for a therapeutic index measure this dosage in terms of the amount of the medication in the bloodstream. This allows for a more reliable estimate of dosage between individuals when compared to oral dosages. Additionally, the effect caused by the dose is considered to be an all-or-nothing one. For antihistamine medications, the dose would have to relieve allergy symptoms to be considered effective.

Individuals can use the therapeutic index to learn about the relative safety of a medication. The larger the ratio of toxic to effective dose, the safer a drug is generally considered to be. A large therapeutic ratio means that a person would have to take many times the effective dose in order to experience harmful effects.

Drugs with a smaller therapeutic index may still be used, but are often only taken under a doctor's supervision. Determining the proper dose for one of these drugs may have to factor in several aspects of the person taking it, such as weight, kidney function, and other medications taken. While taking a medication with a small therapeutic ratio, individuals may opt to have their plasma levels monitored, so that they can make sure that the concentration of the drug in their bloodstream stays at a safe amount.

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donasmrs
Post 3

If a drug has such a small therapeutic index that it's difficult to determine the dose that will have benefits, but without dangerous side effects, then drug probably shouldn't be used.

As far as I know, different people taking the same dose of a drug may have varying levels of the drug in their blood. People's bodies absorb and process drugs at different rates and other factors can affect absorption. So I think this is a risk that's not worth taking if a drug has severe side effects at slightly higher doses.

ddljohn
Post 2

@ysmina-- That's a good question. You might want to ask your doctor this at your next visit.

I'm not a doctor but I think that doctors don't mention this if they know that a medication has a wide therapeutic index. Meaning that the patient would have to take a lot of the drug to experience negative effects. And since doctors specifically tell their patients what dose to take and trust that their patients will follow directions, it's not a concern to most.

If a drug has a very narrow therapeutic index, then the patient should certainly be warned and told never to take more than the recommended dose. There are some drugs like that on the market, like warfarin for example.

ysmina
Post 1

I had never heard of this before. It's not something that my doctors have ever mentioned to me. If it's so important to know his when using medications, why don't doctors warn us?

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