Drugs and other physical treatments

Drugs and other physical treatments

Dr. Safeya Alchalabi

The pharmacokinetics of psychotropic drugs

● absorption
● distribution
● metabolism
● excretion.

The pharmacokinetics of psychotropic drugs

Plasma half-life
The half-life of a drug in plasma is the time taken for its concentration to fall by a half, once dosing has ceased.
• For most psychotropic drugs, the amount eliminated over time is proportional to the plasma concentration, and in this case it will take approximately five times the half-life for the drug to be eliminated from plasma.

Pharmacodynamic measures

positron emission tomography (PET) can be used to measure directly the degree of brain dopamine-receptor blockade produced by antipsychotic drugs during treatment.

Drug interactions

When two psychotropic drugs are given together, one may interfere with or enhance the actions of the other.


Drug interactions
Interference may arise through alterations in absorption, binding, metabolism, or excretion 
(pharmacokinetic interactions),
or by interaction between the pharmacological mechanisms of action 
(pharmacodynamic interactions).

Drug interactions

Pharmacokinetic interactions
Interactions that affect drug absorption are seldom important for psychotropic drugs, although it is worth noting that absorption of chlorpromazine is reduced by antacids.
Interactions due to protein binding are also uncommon, even though many psychotropic drugs are highly protein-bound.
Interactions that affect drug metabolism are of considerable importance.
Interactions that affect renal excretion are mainly important for lithium, the elimination of which is decreased by thiazide diuretics.

Drug interactions

Cytochrome P450 enzymes.
These enzymes are located mainly in the liver but also in other tissues, including the gut wall and brain.
Their role is to detoxify exogenous substances such as drugs,
their activity can be increased or decreased by concomitant drug administration.
Importantly, several antidepressants, particularly SSRIs, potently inhibit P450 enzymes.

Drug interactions

Pharmacodynamic interactions
Pharmacodynamic interactions are exemplified by the serotonin syndrome, in which drugs that potentiate brain 5-HT function by different mechanisms (e.g. SSRIs and MAOIs) can combine to produce dangerous 5-HT toxicity.


Drug withdrawal
Psychotropic and, indeed, many other classes of drugs produce neuroadaptive changes during repeated administration.

Drug withdrawal

Tissues therefore have to readjust when drug treatment is stopped; this readjustment may appear clinically as a withdrawal or abstinence syndrome.

Drug withdrawal

Characteristic abstinence syndromes have been described for antidepressants, antipsychotics, and anxiolytics, while sudden discontinuation of lithium can provoke a ‘rebound’ mania in patients with bipolar disorder.

Drug withdrawal

It is important to be able to distinguish withdrawal syndromes from relapse of the disorder that is being treated.

Drug withdrawal

In addition, the risk of abstinence symptoms makes it prudent to withdraw psychotropic drugs slowly wherever possible.

General advice about prescribing

Use well-tried drugs
Give an adequate dose
Use drug combinations cautiously
Dosing and treatment duration
What patients want to know
Compliance, concordance, and collaboration
Ethical aspects of drug prescription


General advice about prescribing
What patients want to know
what the drug is being used for?
what therapeutic effects are expected?
when they should start to appear?

General advice about prescribing

What patients want to know
• Other key questions that must be dealt with include the following:
● What effects will I experience on first taking the drug?
● What side effects can I expect?
● What serious side effects should I report immediately?
● For how long should I take the drug?
● Is the drug addictive?
● What will I notice when I stop the drug?

General advice about prescribing

Compliance, concordance, and collaboration
If patients are to take medication reliably, they must be convinced of the need to take it, be free from unfounded fears about its dangers, and be aware of how to take it.

General advice about prescribing

Compliance, concordance, and collaboration
Thus patients with schizophrenia or seriously depressed patients may not be convinced that they are ill, or they may not wish to recover.
Deluded patients may distrust clinical staff, and
hypochondriacal patients may fear dangerous side effects.
Anxious patients often forget the prescribed dosage and frequency of their drugs.


General advice about prescribing
Compliance, concordance, and collaboration
Written instructions and information can be a valuable adjunct, and are now included with drug packaging.

General advice about prescribing

Compliance, concordance, and collaboration
it is clearly important to recognize that the use of drug treatment, particularly in psychiatry, requires a thorough understanding of the patient’s attitude to both their illness and its treatment.

Prescribing for special groups

Children and the elderly
In the case of antidepressants, for example, only fluoxetine has a licence for the treatment of depression in young people.
Elderly patients are often sensitive to side effects of medication, and may have impaired renal or hepatic function; for these reasons it is important to start treatment with low doses.

Prescribing for special groups

Pregnant women
Anxiolytics and antidepressants
Antipsychotic drugs and mood stabilizers

Prescribing for special groups

Neonatal toxicity


Prescribing for special groups
Breastfeeding

What to do if there is no therapeutic response

1. Is the drug being taken as recommended?
2. Is the patient taking any other drug that could affect the metabolism or pharmacological action of the psychotropic agent?
3. Is the diagnosis correct?

Thank you