Can a single dose of poison applied to an edged weapon be used multiple times?

Question

If a fighter applies a single dose of regular poison to his sword, but he can make two attacks per round, does that mean that he can make twenty attacks (2 attacks × 10 actions/minute) with poison damage before the poison dries? Or does it only mean that he must make his attack within one minute of applying the poison, and after that single attack the poison is lost?

Answer 1

The poison remains until piercing/slashing damage is done, or until it is washed off.

This appears to have been officially errata'd in 2015. Here is the relevant portion with the relevant sentence emphasized:

Poison (p. 257). [...]

The other three poison types have new descriptions:

[...]

Injury. Injury poison can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage and remains potent until delivered through a wound or washed off. A creature that takes piercing or slashing damage from an object coated with the poison is exposed to its effects.

Thus, after the poison deals its effect once, it’s no longer potent.

The other poison types have new descriptions as well, for those interested.

I suspect the removal of a time limit is for speed of play/simplicity.

Answer 2

What is Regular Poison?

There is no such thing as Regular Poison mentioned anywhere in the PHB or the DMG. Judging by the description the OP uses, the term Regular Poison in the question is most likely referring to Basic Poison which is detailed on PHB page 152:

Poison, Basic. You can use the poison in this vial to coat one slashing or piercing weapon or up to three pieces of ammunition. Applying the poison takes an action. A creature hit by the poisoned weapon or ammunition must make a DC 10 Constitution saving throw or take 1d4 poison damage. Once applied, the poison retains potency for 1 minute before drying.

As of March 2019, no errata has been issued concerning this entry in the PHB.

There are four categories for poisons described in the DMG: Contact, Ingested, Inhaled and Injury. The Injury poison category most closely match the poison mentioned by the OP. Here is the entry for the Injury type of poison on DMG 257 (as updated by DMG errata November 2018, entry on Poison):

Injury. Injury poison can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage and remains potent until delivered through a wound or washed off. A creature that takes piercing or slashing damage from an object coated with the poison is exposed to its effects.

Not All Poisons Are Part of the New Errata.

The errata issued on poisons concerns the poison categories in the DMG. The material in the PHB defining Basic Poison is clearly distinct from the information in the DMG and related errata. The PHB provides information on the usage of one specific poison, whereas the DMG provides information on the usage of general categories of poisons. This is a clear case of specific beats general, where the use of Regular Poison (or Basic Poison as it is rightly called) trumps the rules on poison categories.

However there are two possible answers here, and each answer depends on what the OP means by Regular Poison.

If Regular Poison is a Generic Injury Poison

The description the OP provides for the Regular Poison (if we were to assume for a moment it is not Basic Poison) places it squarely in the Injury Poison category. Mainly because it is being used on a sword, which is a slashing weapon.

For a generic Injury Poison the DMG errata (quoted above) states that once the poison is applied it remains potent until delivered through a wound or washed off. This is quite straightforward. If a fighter applies the poison to their sword the poison is active until damage is inflicted. Then the poison is no longer active.

In this case the fighter can make one damage inflicting attack that would include the additional poison damage. (What that extra damage is only the OP knows, because there is no description of Regular Poison in the DMG or PHB.)

If Regular Poison is Indeed Basic Poison

It is strongly implied by the OP's wording that Regular Poison is indeed the Basic Poison which is detailed in the PHB (quoted above). The description the OP provides includes details of Basic Poison from the PHB, including:

• A single dose of the poison lasts for one minute
• A single dose of the poison lasts until it dries
• Only poison damage is mentioned, the OP makes no mention of other debilitating effects which each of the poisons in the DMG do have.
• The names are very similar and this could easily have been a misquote from the PHB. For the most part, Regular = Basic.

Even if the OP did not intend for Regular Poison to mean Basic Poison, the usage stipulations appear to be equivalent and the one could be replaced for the other when determining rules around usage.

The rules for Basic Poison are clear: The poison lasts for one minute at which point it dries and becomes ineffective. That is all the rules say on potency. The rules do not say that the poison loses potency after inflicting damage.

In this case the regular Poison will maintain it's potency for as many attacks in one minute that hit with the poisoned weapon.

It is worth noting that in this case the DMG errata on Poison Categories (quoted above) has no impact on this ruling. The category descriptions do not affect the usage of Basic Poison because Basic Poison has its own usage rules that are distinct from any of the poison category rules.

Isn't Basic Poison Also an Injury Poison?

No.

There are several clear distinctions between the rules for Basic Poison and the Injury Poison category which exclude the former from the latter:

• Injury Poisons inflict damage if the weapon causes damage whereas Basic Poison inflicts damage when the weapon hits. This is a huge difference and excludes Basic Poison from the Injury Poison category. (For example: Basic Poison would inflict damage if a PC hits a target with resistance to slashing, even if the damage is 0, whereas Injury Poisons must cause damage and would not inflict damage in this case).
• Injury Poisons remain potent until delivered through a wound or washed off. Again Basic Poison deviates from this description because the poison retains potency for 1 minute before drying, regardless of actions that have been taken in that time.
• Injury Poisons can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage. Basic Poison can only be applied to a slashing or piercing weapon or up to three pieces of ammunition. Another huge difference - this also excludes Basic Poison from the Injury Poison category. This difference is telling, because how useful would Basic Poison be in a trap if the poison only lasts one minute? That is why traps are excluded from the description of Basic Poison.

These 3 distinctions clearly exclude Basic Poison from the Injury Poison category. Basic Poison falls outside all four listed categories in the DMG and therefor acts by its own rules. The rules for Basic Poison are listed in the PHB and are not affected by the errata to the DMG released as of March 2019 because Basic Poison is not part of any category edited in that errata.

... And the Contact Poison Category?

Lastly, since Basic Poison renders its effects on a hit (but not necessarily with damage), which is akin to a weapon being in contact with an opponent, it is worth noting that Basic Poison is also distinct from the Contact Poison category. Basic Poison can be applied to a slashing or piercing weapon or up to three pieces of ammunition and retains potency for one minute whereas (DMG 257, per November 2018 errata on Poison):

Contact poison can be smeared on an object and remains potent until it is touched or washed off. A creature that touches contact poison with exposed skin suffers its effects.

The difference being the items that can be coated with the poison and also the conditions for loss of potency. These differences exclude Basic Poison from the Contact Poison category, so the rules of this category have no impact on the use of this poison.

This position is supported by Crawford

For what it's worth, Jeremy Crawford clearly agrees with this assessment here:

The poison in the PH does what it says, and the poisons in the DMG do what they say. One has no bearing on the other.

Answer 3

You gain the effect of an injury poison continuously while the poison remains potent.

From the catalogue of poison types (DMG p. 257):

Injury. A creature that takes slashing or piercing damage from a weapon or piece of ammunition coated with injury poison is exposed to its effects.

In the case of Basic Poison (PH p.153) the duration is quite unambiguous:

Once applied, the poison retains potency for 1 minute before drying.

There is no text anywhere to suggest that a successful attack removes the potency. However, the Word of God is that this may change in future errata.