The #1 reason Americans don't take their medications as prescribed: cost.
Is that true?
Not sure this is good enough for an answer but, generally no.
Patient Education, Side effects, and other concerns usually trump costs. I can't find a nice pie chart to lay out this fact for you but take a look at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3068890/
It lists some of the largest concerns with patients continuing to take medications as prescribed. Their most mentioned one is Patient Education (medical literacy). But they also list problems with the medical system in general, lack of patient engagement, and the fact that if you have to take a pill forever you're just going to forget from time to time.
To @tim's point. This data doesn't really cover the question directly. It only shows that there are other concerns in some cases. I will also add an excerpt from http://blog.ensocare.com/trend-reports/medication-noncompliance-why-wont-patients-take-their-meds
The number of Americans not taking their medications properly is staggering. According to the National Council for Patient Information and Education, about half of the 2 billion prescriptions filled each year in the U.S. are not filled or taken correctly:
- 12 percent don’t fill their prescription at all
- 12 percent don’t take medication at all after they fill the prescription
- 29 percent stop taking their medication before it runs out
- 22 percent take less of the medication than is prescribed on the label
While not a direct answer is shows that of the people not filling or taking their prescriptions correctly 63% have already paid for the prescription. It also shows that 29% (the number one spot on this list) stop taking their medication before the prescription runs out.
However you should note that it doesn't say why 12% of the prescriptions are never filled.