Increased Sensitivity to Many Drugs: The problems of decreased body size, altered body composition (more fat, less water), and decreased liver and kidney function cause many drugs to accumulate in older people’s bodies at dangerously higher levels and for longer times than in younger people.
The elderly are particularly at increased risk of adverse drug reactions (ADR)  attributed in the main to polypharmacy and physiological changes affecting the pharmacokinetics and pharmacodynamics of many drugs or poor compliance due to cognitive impairment or depression.
Type B ADRs are usually uncommon, but rarely may sometimes cause serious toxicities.  Therefore, ADRs in elderly are largely contributed by prescribing error e.g., large doses of drugs without taking into account, the effect of age and frailty on drug disposition, especially renal and hepatic clearance.
Factors which might increase the possibility of the occurrence of ADRs include; extremes of age, gender, multiple drugs, disease state, past history of ADR or allergy, genetic factors, large doses and many other factors.
Amongst the known risk factors for adverse reactions are increasing age, polypharmacy, liver and renal disease as well as being female. Female patients have a 1.5- to 1.7-fold greater risk of developing an ADR, including adverse skin reactions, compared with male patients.
Adverse drug reactions (ADRs) are common in older adults, with falls, orthostatic hypotension, delirium, renal failure, gastrointestinal and intracranial bleeding being amongst the most common clinical manifestations.
These physiological changes include increased body fat, decreased body water, decreased muscle mass, and changes in renal and liver function and in the Central Nervous System. These changes can cause adverse drug reactions (ADRs) in older people.
An adverse drug event (ADE) is when someone is harmed by a medicine. Older adults (65 years or older) visit emergency departments almost 450,000 times each year, more than twice as often as younger persons.
An adverse drug reaction (ADR) is an injury caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs.
Drug-condition interactions may occur when an existing medical condition makes certain drugs potentially harmful. For example, if you have high blood pressure you could experience an unwanted reaction if you take a nasal decongestant.
Factors influencing drug effects
Patients in a hospital’s perioperative setting, including outpatient surgery, the perioperative holding area, the operating room and the post-anesthesia care unit, are at an increased risk of medication errors due to handoffs and lack of communication, according to a report released this week.
Using anticoagulants safely requires a careful balance between risks and benefits. Among older adults, oral anticoagulants are the most common causes of adverse drug events (ADEs) leading to emergency room visits and emergent hospitalizations.
In the elderly there is a reduction in gastric pH which, in the case of some drugs, affects the solubility and thus will influence the rate of absorption. Furthermore, there is a reduction in intestinal blood flow, which would tend to delay or reduce drug absorption.