I know that this is simply an exercise but I think it points out my epistemic belief that there are no such things as gray areas in deceit.

The doctor could have easily have said "It's the weakest painkiller I have," or "It's the strongest sedative I have," or any other number of technically true but misleading statements

In reality the Saline IV is neither a painkiller nor a sedative whatsoever therefore:

The strongest painkiller I have

is actually as much of a lie as the morphine statement.

Now of course you go on to say this:

Because language does not specify single, clear meanings...

However in this case there is a clear meaning for "painkiller" or "sedative."

I think there are good examples of shaping perceptions (which is what NTL's really seem to be) on these types of issues. Here is an alternative for your example. The doctor could have said this:

"This is something which might help your pain"

That is both technically true and verifiable as not NTL. Does it have the same effect as making a definitive statement? Perhaps not, but it also does not lead to a false conclusion about morphine or the ethics of the doctor which the patient may make when he is slicing and dicing.

If lying is never justified because of its effect on the listener, then neither is NTL.

To which I would agree, but I pose another question: Is shaping another person's perception in any way, based on reasonable knowledge of the outcomes and given authority (Such as the case with doctors) a positive heuristic?