A paradoxical reaction or paradoxical effect is an effect of a chemical substance, typically a medical drug, that is opposite to what would usually be expected. An example of a paradoxical reaction is pain caused by a pain relief medication.
Paradoxical reactions are more commonly observed in people with attention deficit hyperactivity disorder (ADHD).
Amphetamines are a class of psychoactive drugs that are stimulants. Paradoxical drowsiness can sometimes occur in adults.
The paradoxical effect or Eagle effect (named after H. Eagle who first described it) refers to an observation of an increase in survivors, seen when testing the activity of an antimicrobial agent. Initially when an antibiotic agent is added to a culture media, the number of bacteria that survive drops, as one would expect. But after increasing the concentration beyond a certain point, the number of bacteria that survive, paradoxically, increases.
In rare cases antidepressants can make users obsessively violent or have suicidal compulsions, which is in marked contrast to their intended effect. This can be regarded as a paradoxical reaction but, especially in the case of suicide, may in at least some cases be merely due to differing rates of effect with respect to different symptoms of depression: If generalised overinhibition of a patient’s actions enters remission before that patient’s dysphoria does and if the patient was already suicidal but too depressed to act on their inclinations, the patient may find themselves in the situation of being both still dysphoric enough to want to commit suicide but newly free of endogenous barriers against doing so. Children and adolescents are more sensitive to paradoxical reactions of self-harm and suicidal ideation while taking antidepressants but cases are still very rare.
Chlorpromazine, an antipsychotic and antiemetic drug, which is classed as a “major” tranquilizer may cause paradoxical effects such as agitation, excitement, insomnia, bizarre dreams, aggravation of psychotic symptoms and toxic confusional states.
Phenobarbital can cause hyperactivity in children. This may follow after a small dose of 20 mg, on condition of no phenobarbital administered in previous days. Prerequisity for this reaction is a continued sense of tension. The mechanism of action is not known, but it may be started by the anxiolytic action of the phenobarbital.
Benzodiazepines, a class of psychoactive drugs called the “minor” tranquilisers, have varying hypnotic, sedative, anxiolytic, anticonvulsant, and muscle relaxing properties, but they may create the exact opposite effects. Susceptible individuals may respond to benzodiazepine treatment with an increase in anxiety, aggressiveness, agitation, confusion, disinhibition, loss of impulse control, talkativeness, violent behaviour, and even convulsions. Paradoxical adverse effects may even lead to criminal behaviour. Severe behavioural changes resulting from benzodiazepines have been reported including mania, schizophrenia, anger, impulsivity, and hypomania.
Paradoxical rage reactions due to benzodiazepines occur as a result of an altered level of consciousness, which generates automatic behaviours, anterograde amnesia and uninhibited aggression. These aggressive reactions may be caused by a disinhibiting serotonergic mechanism.
Paradoxical effects of benzodiazepines appear to be dose related, that is, likelier to occur with higher doses.
In a letter to the British Medical Journal, it was reported that a high proportion of parents referred for actual or threatened child abuse were taking medication at the time, often a combination of benzodiazepines and tricyclic antidepressants. Many mothers described that instead of feeling less anxious or depressed, they became more hostile and openly aggressive towards the child as well as to other family members while consuming tranquilizers. The author warned that environmental or social stresses such as difficulty coping with a crying baby combined with the effects of tranquilisers may precipitate a child abuse event.
Self aggression has been reported and also demonstrated in laboratory conditions in a clinical study. Diazepam was found to increase people’s willingness to harm themselves.
Benzodiazepines can sometimes cause a paradoxical worsening of EEG readings in patients with seizure disorders.
Barbiturates such as pentobarbital have been shown to cause paradoxical hyperactivity in an estimated 1% of children, who display symptoms similar to the hyperactive-impulsive subtype of attention deficit hyperactivity disorder. Intravenous caffeine administration can return these patients’ behaviour to baseline levels.
The mechanism of a paradoxical reaction has as yet (2019) not been fully clarified, in no small part due to the fact that signal transfer of single neurons in subcortical areas of the human brain is usually not accessible.
There are, however, multiple indications that paradoxical reactions upon – for example – benzodiazepines, barbiturates, inhalational anaesthetics, propofol, neurosteroids, and alcohol are associated with structural deviations of GABAA receptors. The combination of the five subunits of the receptor (see image) can be altered in such a way that for example the receptor’s response to GABA remains unchanged but the response to one of the named substances is dramatically different from the normal one.
There are estimates that about 2-3% of the general population may suffer from serious emotional disorders due to such receptor deviations, with up to 20% suffering from moderate disorders of this kind. It is generally assumed that the receptor alterations are, at least partly, due to genetic and also epigenetic deviations. There are indication that the latter may be triggered by, among other factors, social stress or occupational burnout.