According to a recent study of Medicare beneficiaries

... who were provided medication therapy management services, 70% reported taking medications “all of the time”. The remaining 30% provided the following reasons for not adhering to their prescribed medications:

• 73% of them cited forgetfulness
• 11% cited side effects
• 10% said the medication was not needed

Source: "Medication adherence behaviors of Medicare beneficiaries," Patient Preference and Adherence, abstract only, September 17, 2014, http://www.dovepress.com/medication-adherence-behaviors-of-medicare-beneficiaries-peer-reviewed-article-PPA