Have three editors tackled a paragraph that is devoid of commas, and you will probably end up with three pieces of text in which commas are used very differently.
“The American Medical Association Manual of Style” sums up the issue quite nicely:
“There are definite rules for using commas; however, usage is often subjective. Some writers and editors use the comma frequently to indicate what they see as a natural pause in the flow of words, but commas can be overused. The trend is to use them sparingly.”
So keeping in mind that comma use can be subjective, here are five general guidelines about its use:
1. Use a comma after opening dependent clauses or long adverbial phrases.
Here’s an example: “If the infection recurs within two weeks, an additional course of antibiotics should be given.”
However, a comma is not needed if the introductory phrase is short, such as in this case: “In some patients the medication causes sleepiness.”
2. Use a comma to avoid ambiguous or awkward word placement.
• “Patients who can exercise, try to do so every day.”
• “Outside, the ambulance siren shrieked.”