Insertions in Quotations.
Brackets are used to indicate editorial interpolation within a quotation and to enclose corrections, explanations, or comments in material that is quoted. (See also 8.6.1, Quotation Marks, Quotations; 8.8.6, Ellipses, Change in Capitalization; and 8.8.7, Ellipses, Omission of Ellipses.)
“Enough questions had arisen [these are not described] to warrant medical consultation.”
Thompson stated, “Because of the patient’s preferences, surgery was absolutely contraindicated [italics added].”
“The following year  was a turning point.”
Note: Use sic (Latin for “thus” or “so”) in brackets to indicate an error or peculiarity in the spelling or grammar of the preceding word in the original source of the quotation. As with apologetic quotation marks (see 8.6.8, Quotation Marks, Apologetic Quotation Marks), use sic with discretion.
“The plural [sic] cavity was filled with fluid.”
“Breathing of the gas is often followed by extraordinary fits of extacy [sic].”
Use brackets to indicate parenthetical expressions within parenthetical expressions.
In scientific text, one often encounters complex parenthetical constructions such as consecutive parentheses and brackets within parentheses.
A nitrogen mustard (mechlorethamine hydrochloride [Mustargen]) was one of the drugs used.
Her platelet count was 100 000/mm3 (100 × 109/L) (reference range, 150 000 to 450 000/mm3 [150 to 450 × 109/L]).
In mathematical formulas, parentheses are generally used for the innermost units, with parentheses changed to brackets when the formula is parenthetical. (See also 21.3, Mathematical Composition, Stacked vs Unstacked.)
t = d(r1 − r2)
The equation suggested by this phenomenon (t = d[r1 − r2]) can be applied in a variety of circumstances.
In chemical formulas, the current trend is to use only parentheses and brackets, making sure that every parenthetical or bracketed expression has an opening and closing parenthesis or bracket symbol. Consult the most recent edition of USP Dictionary of USAN and International Drug Names for drug formulas and The Merck Index for chemical compounds to verify the correct use of parentheses and brackets.
An experimental drug (9-[(2-hydroxy-1-(hydroxymethyl)ethoxymethyl)]guanine) was used to treat the cytomegalovirus retinopathy in patients with AIDS.
If the older style of parentheses, braces, and brackets has been used by the author, retain it. The notation will be readily understood by the author’s intended audience.
When a parenthetical or bracketed insertion in the text contains a mathematical formula in which parentheses or brackets appear, the characters within the formula should be left as given unless that would place 2 identical punctuation symbols (eg, 2 open parentheses) immediately adjacent to each other. To avoid adjacent identical characters, change parentheses to brackets or brackets to parentheses in the formula as needed, working from inside out, starting with parentheses, to brackets, to braces.