Save VS poison, part 2

I was thinking further on the mod I adopted, instead of the standard "save VS poison, or die" rule.  The one thing I did not take in to consideration was additional poisoning, while being already being poisoned.

I was thinking; In the instance where a character was already poisoned, and took further damage, and failed the poison save, what happened? I mulled it over a bit, and then decided on the following;

• Failing future poison saves, while currently poisoned, extends the duration by 1/2 of the original duration, rounded up. For example; a character has been bitten by a 5HD spider, and fails the poison save, he will take 1d6 1d4 damage, each turn, for 5 turns.  The next round, the character is bitten again, and fails another save. Instead of adding 5 turns to the duration, the character will only add 3, for a total of 8 turns.  

As mentioned before, this may not sound like a lot of damage, but consider that this totals 8d6 8d4, or a max of 48 32 additional points of damage, over a period of time.  Additionally, the randomness of the damage each turn will allow the players some time to find a cure for the poisoning.

With the change in how poisoning works, I had to give some consideration to the spell, Neutralize Poison.  My thought is to make this spell accessible at 1st level, instead of 4th, for Clerics & Druids.