In The Atlantic – one of our favourite magazines – featured an insightful piece about the importance of font choice when creating instructional materials on health information. Health agencies and big pharmacies are big on simplicity in their fonts. For example – the CDC has a health materials guide called "Simply Put" which includes guidelines on which fonts and typefaces to be used. The Centers for Medicare, Medicaid and the US National Library of Medicine all think that Times New Roman, Arial or Helvetica should be used – at least 12-14 point.
The article also states what many marketers already know – serif letters, which help the brain can process more quickly should be used for body copy – and of course, they recommend to avoid italics and underlining.
As with all communications – if it's hard to read – it's hard for people to take action – be it medical directions, or even just buying a product.