Never hold urine / Never hold in urine?

"Never hold in urine" means you should go to the bathroom when your body tells you to.  "Never hold urine" means you shouldn't hold urine in your hands

I would interpret "Never hold urine" as "Never hold urine (for example in your hands)" "Never hold in urine" and "Never hold urine in" (both are fine to use) would be interpreted as "Do not suppress the urge to urinate, go when you need to go" There is no "the" because it's a mass uncountable noun. Never drink water (not "the water", unless you are specifically referring to a local body of water like a fountain or tapwater in a specific region) Sweat cools you down (not "the sweat") etc etc

“Hold in urine” implied holding it in your bladder. “Hold urine” sounds more like you are holding it in a bag, or even without a bag.

I would say neither is correct. You have to tell who's urine. So for example "Never hold your urine". But that also isn't very natural. Saying pee or wee is more idiomatic.

I'd say both are good advice but mean different things. the first one implies holding urine in your hands. the second means to prevent urine from coming out of your bladder

I’ve heard it said most often that “you should never hold your urine,” or “never hold your bladder.” Maybe that’s local?

It's a generally weird thing to say. What is the context?

There may be some confusion due to the influence of archaic ways to say the same thing. To “hold water” used to mean having a full bladder. To be “unable to hold water” was to have an incontinent bladder.

Without knowing the context, "never hold in urine" is more likely the correct phrasing, as I'm assuming you're talking about bladder issues (incontinence). The "never hold" vs "never hold in" difference for containers mainly indicates a difference in *rate* or *speed* of drainage. Saying a container "never holds" a liquid means that it's entirely incapable of doing so, the entire volume of liquid moves through it immediately (or at least, it drains so quickly that it can't be used for a particular purpose). This is only the case in pretty extreme cases of incontinence, so is likely not applicable. If a container "never holds in" a liquid, that generally implies that the container doesn't leak so badly as to be entirely useless, but it does leak enough to be an issue. In practice, this distinction isn't rigidly imposed and there are many use cases where either may be applicable. But in the case of incontinence, the latter sounds better to me. As for why "the" isn't necessary, the answer is because you only use "the" to differentiate a specific instance of something from other instances of it. However, in the case of incontinence, the bladder behaves the same for all instances of urine, and so there is no need to indicate a distinction between instances. You might use "the" in this context though if you're giving a step-by-step narrative of how urination works, because in that case you *would* be referring to a specific instance of urine (i.e. following it as it moves through each part of the body).

Hold in

 holding urine

It’s Hold In Urine, unless you’re talking about actually holding piss in your hand.

Hold means to physically handle something. Hold in means to prevent from getting out. Urine functions as a mass noun and thus there is no definite/indefinite in this context.

In more casual speech you’d say “never hold in your pee”

Never hold YOUR urine. (Holding someone else’s is perfectly ok though)

I'd say "never hold urine in."

It's regional. In farming states, "holding urine" is the correct term, but it apparently sounds odd to townies.

Both are correct but have different meanings.