Tachyphylaxis is sometimes used as a term synonymous with tolerance. It describes a decreasing response following consecutive injections made at short intervals (tachy = rapid, phylaxis = protection)
Criteria:
(1) The drug exerted an effect initially (in distinction to a drug that never had an effect).
(2) After repeated doses the effects for that dose is reduced or may disappear.
(3) The loss of potency cannot be explained by another mechanism (drug interaction, drug deterioration, nonadherence, disease progression).
(4) There are other case reports describing tolerance developing in this or a related drug.
Sometimes the deterioration is due to induction of drug metabolism, increased excretion or altered receptor. In some cases the cause is unknown.
In some cases this results in a constant escalation of dose. A person who has developed tolerance to a dose may receive a dose that would be fatal to a person who has never been exposed to the drug before.
