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What Is a Therapeutic Window?

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  • Written By: M. Glass
  • Edited By: A. Joseph
  • Last Modified Date: 21 November 2016
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A therapeutic window describes the point at which a patient receives enough medicine to address his or her complaint without incurring adverse reactions. All medications have side effects, and determining the proper dose is frequently difficult because patients do not always react consistently. Determining the parameters of the therapeutic window becomes vital when working with addictive or potentially toxic drugs.

Too much medication has the potential to be more harmful than the disorder it is supposed to treat. For example, acetaminophen is commonly used to treat pain and fever but might cause severe liver damage when taken in excess. Properly identifying the therapeutic window also is essential for avoiding addiction. This is particularly problematic for pain management, because many analgesics are addictive.

Despite the risks of over-medicating, under-medicating can be equally problematic. Not only do insufficient dosages fail to resolve the target condition, they also can intensify the problem. For example, under-medicating with antibiotics can result in a stronger infection that is ultimately more difficult to treat than the initial condition was. As drugs are developed and recommended for new conditions, proper identification of the therapeutic window is essential for determining safe and effective dosage recommendations.

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The timing and duration of treatment can also be understood as part of the therapeutic window. Some illnesses, such as shingles, respond to treatment only within a few hours of infection. Similarly, certain conditions, such as stroke or spinal injury, feature a natural therapeutic window for recovery, within which patients can respond to treatment. Some medications have been suggested to extend the window, especially for stroke victims, but with or without medication, speech and physical therapists often have a limited amount of time to effectively help patients recover. After the nerve damage is past a certain point, further recovery is often minimal.

Understanding the width of the therapeutic window for a needed prescription drug is essential for developing safe and effective treatment plans. Narrow windows feature a much larger risk of harm than wider windows. Patients who have unusual response thresholds are more likely to fail to respond to slightly low dosages, but they might be especially vulnerable to slightly high dosages. The edges of the therapeutic window are intended to buffer against these problems, but sufficient safety zones are not always possible for drugs that have very narrow safe and effective ranges.

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ddljohn
Post 3

@turquoise-- I agree with this. And not all drugs have a narrow therapeutic window. Some are wide enough that doctors can prescribe them at higher doses without worrying about adverse effects.

The reason that it's so difficult to predict whether a patient will have issues with a medication is because there are different factors affecting drug absorption in the body. I, for one, am very sensitive to medications. Smaller doses work very well for me and the regular dose is usually too much and causes side effects. I have experienced this for years and my doctor is aware of this as well. So I usually start a medication at half of the regular dose and if there aren't side effects, my doctor increases it after a few days or a week.

turquoise
Post 2

@ysmina-- Of course doctors know about therapeutic windows of drugs.

I'm not an expert on this topic but I think that mild side effects are not usually a cause of worry. So if a drug is effective, but causes some mild, temporary side effects, then it can be said that the dose is within the therapeutic window. Technically, the only way to know is to measure the levels of the drug in the patient's blood and see if it's within the therapeutic index. Obviously this can't be done on a regular basis. It's reserved for very serious situations where a drug has a high chance of causing serious, permanent damage.

ysmina
Post 1

I wasn't aware that medications are supposed to have a window of dosage where they work without side effects. Most of the medications I have used recently have given me side effects. Does that mean that the dose I was on was not within the therapeutic window of that drug? Why do doctors prescribe such doses then? Do they not know what the therapeutic window is?

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