If you score a critical hit, and Yoy have a poison in the weapon... Doy you roll the posion's dice twice as well?
And If your PC scores a critical hit with a rogue with a weapon with poison using Sneak Attack... Do you roll all twice?
Yes, you roll all of the attack’s damage dice you would have normally rolled for a hit twice, and add them together.
My PC don´t agree 
The believe if the is a saving throw, I shouldn´t roll twice
gautxoriak wrote:
You mean for things similar to a Giant Scorpion attack?
Sting. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature.
Hit: 7 (1d10 + 2) piercing damage, and the target must make
a DC 12 Constitution saving throw, taking 22 (4d10) poison
damage on a failed save, or half as much damage on a
successful one.
I would double all the dice related to the attack, including poison. It says that on a hit you do X piercing damage, and Y other damage (part of which can be mitigated with a saving throw). Keep the save as normal, and double all the damage related to the attack.
Think of a spell attack where 1/2 may be considered a specific type of damage, and 1/2 may be considered another type. By breaking the damage out in this case, people who are resistent or susceptable to a type of damage can mitigate part of the attack. So if a character is immune to poison then the second part of the critical will have no effect on them. I believe this is the benefit of breaking down the damage.
The most important thing is that you are consistent in your determination. The rules indicate that all the damage is doubled on a critical. Feel free to make house rules.
1eejit wrote:
I think in the case of the Scropion you would just double the damage of the actual physical attack not the secondary effect of the poison, its not pumping in extra poison or anything.
I would rule that if a critical happened and as a byproduct of that, poisoning happens, the creature wouldn't not take double damage from poison. However, they will save with disadvantage.
I don't think poison damage dependant on a saving throw should be rolled again on a crit though.
In the spirit of 'rulings, etc..' I propose this compromise:
On a critical hit, where the attacker does additional damage requring a saving throw, the additional damage is not doubled, however the saving throw is made with disadvantage.
Carl
My rule of thumb would be; if it doesn't deal plus, extra or additional damage and its instead dealt after a saving throw or check is made, then its not rolled again.
IMO something like a flying snake's bite attack should thus be rolled again, but not the one from a giant scorpion's sting attack for exemple.
Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 1 piercing damage plus 7 (3d4) poison damage.
Hit for #d# - (can crit) + save for #d# (cant crit) so the actual HIT can be crited. the follow up effect is an effect and seperate from the hit / crit
Plaguescarred wrote:
lawrencehoy wrote:
Would you also say that the scorpion attack(s) count twice for concentration checks and that the snake always counts once?