Another contributing factor to an ADE is physician lack of knowledge with regard to the principles of geriatric prescribing.37,52,91 New xenobiotics are sometimes promoted as being safer than older ones, but problems often become apparent after marketing and use by large numbers of patients. For example, the hypnotic agent zolpidem was marketed as a safe alternative to benzodiazepines for the elderly. However, similar to benzodiazepines, zolpidem may also cause confusion, global amnesia, memory loss, and falls.115,117 Low-molecular-weight heparins (LMWHs), such as enoxaparin and dalteparin, are other examples. LMWHs have more predictable pharmacokinetics than unfractionated heparin and are associated with a lower rate of overall bleeding. However, because therapeutic monitoring requires measurement of antifactor Xa activity, which is not as readily available as standard tests such as activated partial thromboplastin time (aPTT), they have not been recommended for routine use.54 LMWHs such as enoxaparin and dalteparin are eliminated by the kidneys, and repeated doses lead to progressive increases in antifactor Xa activity when creatinine clearance is 30 mL/min or below,21,76 a degree of renal insufficiency that is common in frail elderly patients. Less severe levels of renal impairment may also result in reduced enoxaparin clearance that might be avoided with lowered doses.56 Most reported cases of serious, unexpected enoxaparin-induced bleeding, occur in elderly patients who are receiving "standard," not age-appropriate, dosing.83,110,112