Well first thing is first and firstly first. Only puppeteers and medical ninja are aloud to create poisons, others will either need to buy them in a shop/win them in a contest or earn then via mission. Also note all poisons can be healed by certain medical jutsu
Next...
D rank poison
They should have mild effects that should go away in no more than 2 posts, non lethal at all
C rank poison
The effects of these poisons have a bit more of effect and can last up to 4 posts. Hardly lethal
B rank poison
These poisons are more effective than D and C rank and can last up to 5 posts, the effects could be more serious.
A rank poison
These poisons are very effective and can be lethal if left un-treated, these can last up to 6 posts these can be lethal
S rank poison
These poisons are very lethal and may have permanent effects if left un treated effects last up to 7 posts or permanently depending on the effects