At some point in game player gets "Dehydration" as an active effect = Health and stamina are slowly reduced, this I've set as an Ability. There are adequate food/water sources around but at a stage in the quest player begins a lengthy conversation with a NPC. I've tried this myself twice and have died before the conversation tree has finished. Is there a fragment I can put at the beginning of the dialogue that pauses/stops the effect and then resumes at the end? Thanks.

Can you put a condition on it? I know there is a "IsInDialogueWithPlayer" condition, which won't do for your case, but perhaps there is a condition for the Player being in dialogue that you could put on it.

If there isn't a simple condition such as that mentioned by Sundance, could you set up a global variable, which is set to 1 when the player is in dialogue. Once out of dialogue, the Global Variable is set back to 0.

Then on the dehydration ability, place a condition which checks for your global variable.

Placing a condition on the Ability (Spell) means that the condition is checked every second. Its upto you whether this is acceptable.