Prescribing in the Eldery

Polypharmacy- The combination of >=5 drugs (including over the counter and herbal) in one patient.

Elderly patients are often on many drugs, some of which may have been prescribed mistakenly for manifestations of the aging process.  In assessing an elderly patient, it is important to assess the risk-benefit ratio for every drug they are taken, with special consideration of drug interactions.  Common examples of drug interactions include:

  • Antihypertensive medications: almost every antihypertensive medication will interact with another (regularly in a synergistic fashion)
  • Enzyme inducers (e.g. phenytoin, carbemazapine, rifampicin): these will increase the ability of liver enzymes in metabolising other drugs cleared by the same pathway.  A particularly important pathway is the cytochrome p450 3A4 enzyme pathway.
  • Enzyme Inhibitors (e.g. cimetidine; erythromycin): have the opposite effect, and may enhance the effect of other drugs due to an increased plasma concentration secondary to reduced metabolism
  • It is important to consider removing prophylactic medications where the risk from unwanted side effects is greater than the protective effect.
  • Also avoid treating side effects with more drugs, and attempt to treat more than one condition with one drug e.g. Ca channel blocker for hypertension and angina.

Routes of administration

Some older patients may have difficulty swallowing (dysphagia) or they may not want to be swallowing 12 tablet every morning.  If there are alternative routes available (e.g. liquid/syrup, patches, etc), it should be considered.  Similarly, older patients are much more likely to have oedema and the efficacy of patches may be reduced.

Sensitivity

The nervous system of elderly patients is much more sensitive to drugs, particularly centrally acting ones e.g. opioids, BZDs, antipsychotics, antiparkinsonian drugs etc, which should, therefore, be used with more caution.  Similarly, the CVS and other systems can be more sensitive to drugs e.g. antihypertensives.  Where patients have been on such drugs long-term, it may be sensible to reduce the dose.

  • NB Benzodiazepines are the most common cause of adverse side effects in older people

Pharmacokinetics

  • Decreased renal clearance: decreased excretion of renally excreted drugs and a high susceptibility to nephrotoxic drugs (consider especially NSAIDs).  Acute illness can worsen this dramatically.
    • NB Serum creatinine is NOT an accurate representation of kidney function in older people (often the eGFR overestimated in older patients)
  • Decreased liver metabolism of lipid soluble drugs (e.g. opioids/antidepressants) and decreased liver size.

NB Both of these can cause disruption of plasma levels of drugs which can be problematic, particularly for drugs with a narrow therapeutic window e.g. Digoxin-

    • Digoxin toxicity may present with drowsiness, lethargy, fatigue, neuralgia, headache, confusion, hallucinations; nausea & vomiting, abdo pain, SOB, bradycardia, hypotension, syncope
  • Elderly often have a low albumin but high A-1 AG (albumin carries acidic drugs whereas A-1 AG carries basic drugs).  This means, again, that there will be more free acidic drug in the plasma (e.g. phenytoin) but low free levels of basic drugs
  • Elderly have an increased fat mass and decreased water mass in proportion to body mass- lipophillic drugs will have a higher volume of distribution and a longer half life, while hydrophilic drugs will have a lower volume of distribution

In general,

  • Lower doses achieve same effect in the elderly
    • BUT some effects are decreased
  • BEWARE drugs with narrow therapeutic indexes (<2)- this narrows further with age

Common drug interactions

  1. Diuretics- older people are commonly on diuretics to control blood pressure/oedema.  Any other drug with a potential risk of hyponatraemia/hyperkalaemia can push the body out of balance.  A common example is SSRI’s and hyponatraemia.

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