As more patients use the Internet to answer health-related queries, simplifying medical information is becoming increasingly important. To simplify medical terms when synonyms are unavailable, we must add multi-word explanations. Following a data-driven approach, we conducted two user studies to determine the best formulation for adding explanatory content as parenthetical expressions. Study 1 focused on text with a single difficult term (N=260). We examined the effects of different types of text, types of content in parentheses, difficulty of the explanatory content, and position of the term in the sentence on actual difficulty, perceived difficulty, and reading time. We found significant support that enclosing the difficult term in parentheses is best for difficult text and enclosing the explanation in parentheses is best for simple text. Study 2 (N=116) focused on lists with multiple difficult terms. The same interaction is present although statistically insignificant, but parenthetical insertion can still significantly simplify text.