Polypharmacy may lead to drug interactions, i.e., the alteration of clinical drug responses by the prior or concomitant administration of two or more drugs. Drug interactions may be additive, synergistic, or antagonistic. Furthermore, while drug interactions may be harmful, they also may be beneficial in achieving desired pharmacological and clinical effects.
Approximately 150 antihypertensive21, anti-arrhythmic22, and inotropic23 medications—which are used to treat cardiovascular conditions—interact with alcohol. Pain killers, antihistamines, tranquilizers, anticoagulants, and antidiabetic drugs also react negatively to alcohol (Cadieux, 1989; Lamy, 1988). Interactions can produce additive, synergistic, or inhibitory effects. Alcohol and other drugs may compete for enzyme metabolism in the liver, either negating the effect of the medication or increasing the effect. Depressants such as benzodiazepines are especially affected by alcohol in this manner, as are antipsychotics, some anti-depressants, anti-coagulants, oral hypoglycemics, anti-seizure compounds, and anti-hypertensive agents. Older people who are experiencing this effect may show heightened confusion, memory loss, dizziness, incontinence, and death from a central nervous system shutdown (Pollow, Stoller, Forster, and Duniho, 1994; Lamy, 1988; Briggs, Castleden, and Kraft, 1980). Alcohol and medication use even 10 hours or more apart from each other can significantly alter the effects of the medication in the same manner that multiple medication use can cause interactions (Lazow, 2001).
There are three broad types of drug interactions: pharmacodynamic drug interactions, pharmacokinetic drug interactions, and physical or chemical drug interactions.
Pharmacodynamic drug interactions are usually associated with drug-drug interactions at the therapeutic target or receptor. For example, compounds may compete directly for binding to a common receptor. The administration of two drugs with anticholinergic effects, such as an antiparkinsonian drug (e.g., trihexyphenidyl), and a tricyclic antidepressant (e.g., amitriptyline), may result in excessive anticholinergic effects.
Pharmacokinetic drug interactions are the result of alterations in drug absorption, distribution, metabolism, or elimination. Most pharmacokinetic drug interactions are associated with altered drug metabolism, particularly with cytochromes P450 (CYPs) although other enzyme systems are also involved in pharmacokinetic drug interactions.
Physical or chemical drug interactions are the consequence of the interaction of two drugs, e.g., cholestyramine and warfarin, or with OTC drugs and prescribed drugs, e.g., magnesium-aluminum-based hydroxide antacids and fluoroquinolone antibiotics. Physical or chemical drug interactions often lead to therapeutic failure.