Needed vs. Required

In this case, I'd say the words are interchangeable. I do, however, have a quibble with "may". You really mean "might" because "may" implies permission. Here's the explanation I use:

Can, might, may

Use the verb can to describe actions or tasks that the user or application is able to do. Use might to express possibility or when the result of an action is unknown or variable. Use may to state that the user has permission to do something.

Examples
System administrators can define dialing plans for a site.
This file contains information that might affect the installation.

boron said:

In dehydration, an intravenous fluid may be required (needed?).

When to use required and when needed?

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'Required' is stronger than 'needed'.

e.g., "You need to clean up your room." Indefinite time frame, no implied consequences if you don't. "I require you to clean up your room." Still no time frame specified, but there is an implied "or you'll be grounded, sacrificed at the next New Moon, some other bad thing will happen.

So, for your example I'd choose needed, mainly because the extent of the dehydration is unspecified and there may be other medical conditions present that contraindicate intravenous application of fluids.

I treat them as synonyms of each other. When I was a technical author, I probably favoured 'required' because it sounded less needy, but then, 'needed' is shorter which is sometime desirable in long or complicated sentences. I think you could justify either in your example.

CaroGirl said:

In this case, I'd say the words are interchangeable. I do, however, have a quibble with "may". You really mean "might" because "may" implies permission.

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The "permission" distinction is true when you're distinguishing between "may" and "can," but I don't think it is when you're distinguishing between "may" and "might." After all, "might" IS "may"--in the sense that it is "may" in other tenses or moods. For example, compare "He may be late" (present) vs. "He said he might be late" (past). Or for the same tenses using "may" in the permission sense, try this: "May I come over?" vs. "He asked if he might come over." No one talks like that anymore in this country, but originally that's where "might" comes from--the past tense or conditional mood (e.g. "If Star Player hadn't been injured, the team might have won") of may. Your dictionary, under either "may" or "might," should have more such examples.

So, because "might" is the same word as "may" in a different tense or mood, people, especially Americans, have gotten in the habit of using the words interchangeably regardless of tense. According to the dictionary, either one can express possibility or permission--"He may well win" shows that "may" is not limited to expressing permission--and I'm not aware that either one is preferred or "more correct" compared to the other.

boron said:

Duncan,
may I judge by myself when something is needed or required, like

"In severe dehydration, infusion might be required".

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Both may and might have identical first definitions (as per dictionary.com) of auxillary verbs used to express a possibility, so it is up to you to decide which you would prefer. The subtext that comes along with the secondary and tertiary definitions/usages can influence your choices.

boron said:

Does might and required go together at all?

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No, there is no connection between might and required as say, there is a conncetion between either-or and neither-nor.

In ordinary parlance, the two words are essentially synonymous. "Required" is probably more precise in technical work, implying some form of technical or legal requirement.

caw