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.
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.