# Tachyphylaxis

Tachyphylaxis (Greek ταχύς, tachys, "rapid", and φύλαξις, phylaxis, "protection") is a medical term describing an acute rapid decrease in response to a drug after its administration.[1] It can occur after an initial dose or after a series of small doses. Increasing the dose of the drug may be able to restore the original response.[2]

## Characteristics

Tachyphylaxis is characterized by the rate sensitivity: the response of the system depends on the rate with which a stimulus is presented. To be specific, a high-intensity prolonged stimulus or often-repeated stimulus may bring about a diminished response also known as desensitization.

### Molecular interaction

In biological sciences, molecular interactions are the physical bases of the operation of the system. The control of the operation, in general, involves interaction of a stimulus molecule with a receptor/enzyme subsystem by, typically, binding to the macromolecule A and causing an activation or an inhibition of the subsystem by forming an activated form of the macromolecule B. The following schematic represents the activity:

${\displaystyle A{\xrightarrow {\ \ p\ \ }}B}$

where p is the activation rate coefficient. It is customary that p is called a rate constant, but, since the p stands for measure of the intensity of the stimulus causing the activation, p may be variable (non-constant).

More complete is an open system, namely, in its simplest form,

${\displaystyle R{\xrightarrow {}}A{\xrightarrow {\ \ p(S)\ \ }}B{\xrightarrow {\ \ q\ \ }},}$

where R stands for the rate of production of A, p(S) is the activation rate coefficient explicitly expressing its dependence on the stimulus intensity S and q represents the rate coefficient of removal from the state B. In this elementally open system the steady state of B always equal to R/q.

The above scheme is only the necessary condition for the rate sensitivity phenomenon, and other pathways of deactivation of B may be considered, with the subsequent return to the inactive form of the receptor/enzyme A. Examples[3][4][5] offer particular use of such (mathematical) models in endocrinology, physiology and pharmacology.

## Examples

### Psychedelics

Psychedelics such as LSD-25 and psilocybin-containing mushrooms demonstrate very rapid tachyphylaxis. In other words, one may be unable to 'trip' two days in a row. Some people are able to 'trip' by taking up to three times the dosage, yet some users may not be able to negate tachyphylaxis at all until a period of days has gone by.

### Opioids

In a patient fully withdrawn from opioids, going back to an intermittent schedule or maintenance dosing protocol, a fraction of the old tolerance level will rapidly develop, usually starting two days after therapy is resumed and, in general, leveling off after day 7. Whether this is caused directly by opioid receptors modified in the past or affecting a change in some metabolic set-point is unclear. Increasing the dose will usually restore efficacy; relatively rapid opioid rotation may also be of use if the increase in tolerance continues.

### Beta-2 agonists

Inhalation of an agonist for the beta-2 adrenergic receptor, such as Salbutamol, Albuterol (US), is the most common treatment for asthma. Polymorphisms of the beta-2 receptor play a role in tachyphylaxis. Expression of the Gly-16 allele (glycine at position 16) results in greater receptor downregulation by endogenous catecholamines at baseline compared to Arg-16. This results in a greater single-use bronchodilator response in individuals homozygous for Arg-16 compared to Gly-16 homozygotes.[6] However, with regular beta-2 agonist use, asthmatic Arg-16 individuals experience a significant decline in bronchodilator response. This decline does not occur in Gly-16 individuals. It has been proposed that the tachyphylactic effect of regular exposure to exogenous beta-2 agonists is more apparent in Arg-16 individuals because their receptors have not been downregulated prior to agonist administration.[7]

### Nicotine

Nicotine may also show tachyphylaxis over the course of a day, although the mechanism of this action is unclear.[8]

## Intervention and reversal

### Intranasal decongestants

Use of intranasal decongestants (such as oxymetazoline) for more than three days leads to tachyphylaxis of response and rebound congestion, caused by alpha-adrenoceptor mediated down-regulation and desensitization of response. Oxymetazoline-induced tachyphylaxis and rebound congestion are reversed by intranasal fluticasone.[10]