Pharmacodynamics refers to what a drug does to the body. The pharmacodynamic effect of a drug is due to changes in specific receptor target sites (e.g. an organ or cell), which depend on the number of receptors, the drug's affinity for the receptors and post receptor events (e.g. impaired activation, amplification or altered response). Older people may have an altered response to drugs because of a decline of homeostatic mechanisms, which usually oppose the effects of drugs in younger people, leading to a higher rate and intensity of adverse effects.

Individuals will vary in which mechanisms are affected most and to what extent. A frail older person may be managing well with homeostatic changes until a drug is taken that impairs their ability to compensate. This may lead to an inability to cope, confusion, falls or hospitalisation. New prescriptions for frail older people must be approached with caution and evaluated.

Orthostatic circulatory response

When resting, blood pressure and heart rate drop. These should return to active levels almost immediately after rising. However, this process is blunted in older people and is a cause of falls and poor mobility.

Older people naturally adapt to this by standing more slowly, but medicines causing postural hypotension can trigger falls. Older people have reduced dopamine receptors in the brain, which impairs the reflexes that maintain posture. An older person's ability to maintain balance depends on their level of physical activity.

Most adapt by being cautious when negotiating hazards. However, if medicines induce drowsiness, this can cause falls. Benzodiazepines cause an increase in postural sway in all age groups.

Such drugs should only be prescribed when necessary for the shortest period possible and should be taken at bedtime. Long-acting preparations should be avoided.

Thirst and hydration

Older people have a reduced sensation of thirst and tend to secrete ADH (a hormone that maintains water balance) inappropriately. This may be seen in older people with chronic cardiac, hepatic or renal disease. Patients taking drugs that cause hyponatremia should be monitored closely.

Thermoregulation

Older people are less efficient at maintaining body temperature. Hypothermia is involved in approximately 4,000 hospital admissions in the UK each winter. Predisposed individuals include those with hypothyroidism, Parkinson's disease and Alzheimer's disease.