Is there a word that means that what was once good for you eventually becomes harmful?

Question

More of an adjective to describe a noun. The example that comes to mind is the umbilical cord feeds and natures but once birth happens doctors sever it. If left too long it could harm the baby. So another way to look at it is something is good but too much of it will be bad. A phrase that comes to mind is a quote from The Dark Knight:

"You either die a hero or you live long enough to see yourself become the villain."

I write poetry and like to include repetitiveness in the sense of words and their definitions if that makes any sense. I am still in the editing part of my poem but here is the part where I would like to insert such word if it exists.

Stop writing encores that sound like the first one, you're just rearranging the words
Give birth to new comings and with them comes what once feed you with essentials
So sever all those ___ umbilical cords

So the word would go in the last line between "those" and "umbilical".

Answer 1

Obsolete may be effective in your context- not exactly the meaning described but goes with umbilical cord well.

Answer 2

"Once good for you and now poisonous" is basically a description of food rot, so words like "rot", "decay", or "mold" get close to what you're going for.

Answer 3

If it's not too dramatic, I would suggest treacherous.

It has the advantage of two meanings that suit your example:

1. If left connected, the umbilical cord would (poetically) turn against, or betray the baby
2. If left connected, it becomes dangerous to the baby

Answer 4

For words that might capture the general, context-neutral “once good, now harmful” part of your question, perhaps you could consider using some poetic license to extend ones that are usually used to describe such people, such as

“turncoat,” “traitorous,” “double-crossing,” etc. ...

... to describe things or concepts that have gone from “good to bad.”

For the specific case of “umbilical cord,” however, I think that using a suitable antonym for an umbilical cord’s particular initial good purposes (life-giving/nurturing) would work better, for example:

So sever all those [now] stifling/smothering/strangling umbilical cords.

(My favorite would be “now strangling” since "strangling" already has a negative association with umbilical cords that you could play off of).

Answer 5

A good option for your specific example may be superfluous. From Oxford Dictionaries Online:

ADJECTIVE
Unnecessary, especially through being more than enough.

I think the "more than enough" connotations make sense for something that used to be necessary, but is now holding you back. In your example this would be:

Stop writing encores that sound like the first one, you're just rearranging the words
Give birth to new comings and with them comes what once feed you with essentials
So sever all those superfluous umbilical cords

This, to me, implies more that the umbilical cord is no longer useful than that it has necessary become actively harmful, though I think the implication of any unnecessary cord is probably that it is likely to trip you up and hold you back.

You could also use the adjective surfeiting here. It's relatively rare, but appropriately poetical and has almost your exact intended meaning. From the Oxford English Dictionary¹, with a couple of relevant attestations:

surfeiting, adj.

1 That causes surfeit; that produces a state of surfeit or satiety. Frequently fig. and in figurative contexts.

1576 T. B_^EDINGFIELD tr. G. Cardano Comforte (new ed.) iii. sig. J.viii^v He desyreth to sleape, whiche is not easlye had his stomacke beinge ouercharged wyth a surfytinge supper.
. . .
1995 Sun Herald (Sydney) (Nexis) 16 July 124 The fugal finale of the Verdi was akin to a tingling sorbet at the end of a refreshing but not surfeiting meal.

Of course this adjective is derived from surfeit, defined by Oxford Dictionaries Online (the free one) as

NOUN (plural surfeits)
1. usually in singular An excessive amount of something.
. . .

VERB (surfeited, surfeits, surfeiting)
[WITH OBJECT] (usually be surfeited with)
1. Cause (someone) to desire no more of something as a result of having consumed or done it to excess.

Shakespeare has a somewhat famous example of using surfeit to mean "too much of a good thing":

If music be the food of love, play on.
Give me excess of it that, surfeiting,
The appetite may sicken, and so die.

—Twelfth Night, 1.1, 1-3

This fits your desired meaning very well: Music, food, drink, and so forth are all good for you in moderation, but if you have a surfeit of them or surfeit on them then they become bad for you. In adjectival terms, we could say that you have had a surfeiting amount, or that the thing itself is surfeiting. In your example it would be

So sever all those surfeiting umbilical cords

Here, the implication is not just that the umbilical cord is holding you back, but that it is continuing to feed you unnecessarily, to the point of being harmful. Note that this would be a slightly different harm, however, than being connected to a rotting placenta post-birth.

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¹ _{"surfeiting, adj." OED Online, Oxford University Press, June 2017, www.oed.com/view/Entry/194903. Note that the OED is different from the above-cited ODO, and unfortunately is a prescription service (behind a paywall). However, many folks, especially in the UK, will have access through their local public library, and most students should be able to access through their institution's library. More info here: http://public.oed.com/about/free-oed/}

Answer 6

Defunct comes close as it broadly means "(something) which ceased to be of value". Interestingly, the M-W definition below also mentions "no longer living" which fits neatly in the context of an umbilical cord.

... So sever all those defunct umbilical cords ...

M-W:

defunct adjective

: no longer living, existing, or functioning

Usage examples with "umbilical cord":

Google search of "defunct umbilical cord"