One PC doing too much damage

I just took over as DM for our group. The previous DM allowed our ranger to purchase a significant amount of Wyvern poison at the end of the last adventure to dip his arrows in. With this poison, the ranger is doing 7d6 poison damage when he hits.

Our characters are all level 5 and 6 so this is obviously a significant amount of damage for this level. I’m worried by simply upping the AC of NPCs, no one else will be able to hit. It feels a little cheap to give the majority of NPCs poison resistance.

Any tips on how to combat this issue?

What I’m considering: Our barbarian has an evil, sentient sword. My thought was while the group is traveling and asleep, the evil sword has become jealous of the amount of damage the range is dealing so the sword mind controls the barbarian to get up and dump most of the poison out the back of the caravan they’ll be traveling in.

Is this too brutal to do to the ranger??