The roots of the Poke Salut plant are deadly poisonous. NEVER EAT THE ROOTS!
The leaves and berries are not poisonous but they have extremely high levels of Vitamin B12. Enough B12 that unless you parboil the leaves twice, they can make you nauseous and have sweats. One to two berries per day are good for arthritis but you don't want to take them on an empty stomach.
If you'll make a homemade wine from the berries, a tablespoon or two per day will work wonders for arthritis , plus it's got all the other benefits of taking a B12 shot.
And like with any home remedies involving wild plants, have someone experienced in identifying and using the plant to teach you