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Pharmacokinetics
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As psychotropic drugs need to be lipophilic to cross the blood brain barrier they are readily absorbed and widely distributed. Almost all are metabolised in the liver to hydrophilic compounds excreted by the kidney. Many have long half-lives and therefore only need to be administered once a day.
Interactions
Many interactions occur between psychotropic drugs and other psychotropics as well as other classes of drugs. It is important that you are aware that both adverse and advantageous interactions can occur. An example of an advantageous interaction may include the use of multiple anticonvulsants in the treatment of epilepsy. There is emerging evidence regarding the use of multiple mood stabilisers, antipsychotics and antidepressants. However these are strictly for use by specialists and the general rule of avoiding polypharmacy should be adhered to as far as possible. The list of interactions below only gives examples - you should consult a source such as the British National Formulary for specific information. Interactions can be divided into:
Pharmacokinetic
These interactions commonly result from induction or competitive inhibition of hepatic enzymes, for example antimuscarinics induce metabolism of phenothiazines and reduce serum level; phenothiazines compete for metabolism with tricyclics and may increase serum levels; some SSRIs inhibit the enzymes that metabolise tricyclics and phenothiazines.
Pharmacodynamic
These interactions occur when compounds act at the same receptor, synapse or system, e.g. alcohol potentiates other sedatives.
Adverse Effects
Important adverse events for specific drug groups are detailed under each of the groups described below. However you should consult a source such as the British National Formulary for more comprehensive information. When considering adverse effects generally, it is helpful to use a simple checklist:
Dose dependent (e.g. via receptor mechanism)
- Autonomic: e.g. Antimuscarinic (phenothiazines, tricyclics) namely: dry mouth, blurred vision, closed angle glaucoma, tachycardia, constipation, ileus, urinary retention.
- Cardiovascular: e.g. Hypotension, Arrhythmia (phenothiazines, tricyclics)
- Psychiatric: e.g. Delirium (phenothiazines, tricyclics)
- Neurological: e.g. Seizures (phenothiazines, tricyclics)
- Endocrine: e.g. Amenorrhoea, weight gain (phenothiazines)
Dose independent (e.g. via autoimmune mechanism)
- Hepatitis: (phenothiazines, tricyclics)
- Blood dyscrasias: (phenothiazines, tricyclics, N.B. clozapine)
- Skin rashes: (phenothiazines, tricyclics)
High risk situations (Pregnancy, breast feeding, the elderly and the physically ill)
Lipophilic drugs readily cross the placenta and also enter breast milk. Teratogenesis (e.g. carbamazepine) and neonatal toxicity (e.g. sedation with benzodiazepines) may occur. Whenever possible psychotropics should be avoided in early pregnancy or when there is a risk of pregnancy and many are contraindicated in late pregnancy or breast-feeding. When they are required their use should only be used under the supervision of a specialist. An evidence based approach can be aided with use of information from local Drug Information Centres or organisations such as Motherisk.
The elderly may have low body mass, reduced hepatic metabolism and a less adaptable central nervous system, rendering them prone to adverse effects (e.g. delirium). Reduced dosages are often advisable. Beware interactions with concurrent physical illness and its treatment.
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