These phrases act as adverbial phrases modifying the verb in the sentence. They indicate the frequency or manner of an action, and they are typically placed after the verb.
Here's why:
* Clarity: A comma before these phrases would create a pause that's unnecessary and could disrupt the flow of the sentence.
* Grammar: In general, you shouldn't use a comma before a phrase that directly modifies the preceding verb.
Example:
* Incorrect: Take the medication, as needed.
* Correct: Take the medication as needed.
Exception:
If the phrase "as needed" or "as directed" is used in a very long sentence or in a complex grammatical construction, you *might* consider a comma for clarity. However, this is rare, and it's best to err on the side of not using a comma in most cases.