Therapeutic effect refers to the responses(s) after a treatment of any kind, the results of which are judged to be desirable and beneficial. This is true whether the result was expected, unexpected, or even an unintended consequence. An adverse effect, including nocebo, on the other hand, refers to harmful or undesired responses(s).
What constitutes a therapeutic effect versus a side effect is a matter of both the nature of the situation and the goals of treatment. No inherent difference separates therapeutic and undesired side effects; both responses are behavioral/physiologic changes that occur as a response to the treatment strategy or agent. However, changes that are viewed as desirable, are called therapeutic.
Many people think of therapeutic and undesired side effects as applying only to medications, drugs or supplements, perhaps because pharmacology approaches are rigorously evaluated by carefully controlled comparisons with placebo treatments, but this is not the case. It applies to any treatment approach, including surgery, physical therapy, psychotherapy, treatment with compounds, faith healing, hypnosis, holistic methods, etc.
The administration of a compound was selected for the pharmacological example below because this form of treatment is often more readily evaluated by comparison with a placebo. Other therapeutic methods are typically more difficult to test because the test subjects can more easily recognize the treatment aspect of interest; it thus becomes far more difficult to apply the placebo control methodology. The placebo effect is relevant in all treatments; behavior and symptoms do change, however the presumed mechanism may primarily be the power of the mind that controls behavior and perception, and the fact that if an individual believes that their situation will change then their situation actually does change.
To maximize therapeutic effects and minimize side effects, recognition and quantification of the treatment, in multiple dimensions, is a critical prerequisite. Many situations allow the effects of a treatment, both those often viewed as both desirable and undesirable to be used in combination with other treatments, so that the best end results actually depend on side effects contributing to the overall result. Achieving this reflects a higher degree of physician and patient interaction, trust, sophistication and skill. The same principles applies to all agents, including commonplaces such as food, water, air and oxygen. Situation, timing and familiarity of the multiple usual responses to agents is critically important for wisely selecting all treatments. Maintaining and improving health strongly depends on promoting desirable effects while lessening the impact of undesirable effects across many interacting issues.
"Natural" or "organic" agents are promoted as more healthy than conventional products. However, every object has multiple and varying responses when used; both desirable and undesirable effects. Even water – on which all life on earth depends – can have undesirable, even fatal effects; while increased intake of water can save a dehydrated patient, too much water can lead to water intoxication, which can result in death, such as in severe pulmonary edema.
As a simple example, the therapeutic effect of diphenhydramine, when used for nasal congestion, is to lessen mucous membrane secretions witih a side effect of drowsiness. However, when used for insomnia, as in many over-the-counter preparations, its therapeutic effect is drowsiness and its side effect is mucous membrane dryness, which is undesirable, especially if the person using the agent for sleep is already suffering from dry membranes.
The desire to effect predictable responses in clinical situations is one of the important reasons that physicians often prefer refined prescription drug preparations, as opposed to less refined products.
Products from nature are typically complex mixtures of large numbers of chemical agents, which complicate understanding their effects.
The pharmaceutical industry purifies, concentrates and sometimes modifies molecules obtained from natural sources, to reduce the variables in the treatment agent.
A pharmaceutical-grade agent makes the effects more definable and predictable. This purification can greatly improve the probability of triggering the desired therapeutic effect.
One of the most basic pharmacological strategies used in evaluating agents is called Therapeutic Index. It can be applied in many ways. At the most basic level, Therapeutic Index is the ratio between the dosage which results in a desired effect for 50% of individuals tested versus the dosage which results in toxicity for 50% of individuals tested. However, this application of Therapeutic Index is focused on basic safety and tolerability. Modifications of this index can be used to compare a variety of responses, e.g. dry mucous membranes for 50% vs. sleepiness for 50%.