What is a common idiom meaning, 'that a recovering patient's medical situation is likely to change all of a sudden without notice or reason'?

The situation to which I refer involves a person who has had an accident and their recovery was going well then not and it keeps changing. I am trying to warn someone caring for them that the situation can change quickly without notice and I can't think of the idiom to describe the speed and unpredictability of that potential change so that they will take nothing for granted.

Solution 1:

I do like the other answers as well. I guess in such a situation, you'll want to be more detailed and sensitive about the condition, but if you're looking for a one word description:

volatile:
MW 1 a: characterized by or subject to rapid or unexpected change

Though correct, this might sound a bit too technical and cold in the context of the health of a human being.

Solution 2:

One phrase that fits well for this situation is touch and go - it means that the outcome of a situation is uncertain, and that things can change at a moment's notice. A person in the hospital after a severe accident may be in "touch and go condition" if it's unsure if they'll survive, and it can be used to highlight the unpredictability of the outcome even if it seems they are getting better. A situation that is "touch and go" can change rapidly and dramatically.

Solution 3:

Medical parlance when talking about someone whose condition is not getting any worse is, "stable." You often hear this in news reports after someone is injured, "Doctors describe the victim's condition as critical but stable."

While it would probably offend a doctor's sensibilities to describe a patient's condition as, "unstable," from someone who wasn't a doctor the meaning would be clear.

Unstable
d(1): liable to change or alteration
MW

A medical professional would probably use the phrase, "Their condition is not yet stable," or, "We are trying to stabilise them," mostly to avoid the implications of the word unstable and it's association with mental health.

Solution 4:

The situation is 'hanging by a thread':

Be in a highly precarious state.

or

to be in a very dangerous situation or state : to be very close to death, failure, etc.
Example: The patient's life was hanging by a thread.

Dictionary.com gives a few more examples, and the origin, emphasizing that this idiom fits your prerequisite of the change being able to happen "without notice or reason":

Also, hang by a hair. Be in a risky or unstable situation, as in His promotion was hanging by a thread, or With the lead actor sick, the success of our play hung by a hair.
This expression, already proverbial in the early 1500s, alludes to Damocles, who vexed King Dionysius with constant flattery. The king invited him to a banquet where Damocles found himself seated under a naked sword suspended by a single hair, symbolizing his insecure position at the court.

Solution 5:

When things can change course rapidly, one expression is "to turn on a dime." In general, the phrase "Life can turn on a dime" or "Life turns on a dime" applies to the big picture to include appreciating what you have and how things can change at any moment. Change can go in different directions, so it's not necessarily a foreboding phrase as much as it marks an abrupt switch. You could say someone's behavior turns on a dime when the changes are sudden. (TFD)