Therapy (in Greek: θεραπεία) is the attempted remediation of a health problem, usually following a diagnosis. In the medical field, it is synonomous with the word "treatment".
A therapeutic effect is a consequence of a medical 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 of the treatment.
What constitutes a therapeutic effect vs. a side effect is a matter of both the nature of the situation in which a treatment is used and the goals of treatment.
In addition to (or in place of) the intended therapeutic effect of a treatment, a therapy may cause undesired (adverse) effects as well. When an adverse effect is weaker than the therapeutic effect, it is commonly referred to as a "side effect". An adverse effect may result from an unsuitable or incorrect dosage or procedure (which could be due to medical error). Some adverse effects occur only when starting, increasing or discontinuing a treatment. Using a drug or other medical intervention which is contraindicated may increase the risk of adverse effects. Patients sometimes quit a therapy because of its adverse effects. The severity of adverse effects ranges from nausea to death. Common adverse effects include alteration in body weight, change in enzyme levels, loss of function, or pathological change detected at the microscopic, macroscopic or physiological level.
Adverse effects may cause a reversible or irreversible change, including an increase or decrease in the susceptibility of the individual to other chemicals, foods, or procedures (e.g. drug interaction).