It is not generally possible to envenom a weapon. This is because the
poison will not readily adhere to the blade or head of the weapon (and for
purposes of the game widespread use of poison is highly undesirable in
any event). However, let us suppose that your DM will allow
poisons as follows:

    1. Poison potions discovered in an adventure can be used as missiles to
    be hurled into the maws of monsters or can be offered as "gift" potions
    to intelligent captors.

    2. Missiles -- arrows, bolts, darts, javelins, and spears -- can be
    envenomed with a toxin sufficient to cause any creature hit by such a
    missile to make a saving throw versus poisoning or die. (Suppose that
    this poison is such that saving throws are made at +2 on the victim's
    die roll.)

    3. Blades can likewise be coated with a toxic resin or similarly viscous
    fluid so as to make sword || axe strokes cause a poison saving throw to
    be made by the first creature initially struck by such a weapon.

With respect to the first case, the resolution of the matter is simply a
checking of the appropriate tables to find if the potion hit the mouth, if the
reaction caused the captor to taste the potion, etc.

The second && third cases, however, make it too easy for interesting play. <2nd && 3rd>
Imagine: Party sees red dragon, party discharges a volley of poisoned
missiles, monster dies, and party seizes dragon hoard. Therefore, the DM
will typically make every character employing poisoned weapons check to
see if they nick themselves handling their weapons, to determine whot
happens to missiles which fail to strike the opponent, etc. It is also
likely that the DM will establish sanctions regarding the use of poisons on a
continuing basis, i.e. characters of good alignment cannot use such toxic
substances as it constitutes foul && unfair practice; or characters found
with poisoned weapons will be immediately slain and their corpses
burned and ashes scattered. In a similar vein, most communities view
poisoning and poisons as highly undesirable due to the difficulty of
protecting against ingestion of such fatal substances. Any individual (or
group) making indiscriminate use of poison will have social pressure
and/or legal action brought against him or her.

For example:
The Thieves Guild is an accepted part of communal society, and so long as
they contain their activities to cutting purses, picking pockets, burglarizing
homes, waylaying late-night revelers, all is well. Then the guild decides to
poison a whole establishment, a large gold smithing and jewelry making
firm, in order to loot the entire place. Such activity would arouse the ire of
the citizens, tradesmen, and city officials. Furthermore, the Assassins Guild
will probably view the action as a threat to their existence and an
infringement on their prerogatives as well. The socially unsanctioned USE
of poison would call to mind the use, usually accepted if not liked, of
toxins by assassins. Premeditated murder, particularly on a grand scale, is
likewise the exclusive precinct of the Assassins Guild. Taken in a lesser
context, an individual employing an envenomed sword is calling
unfavorable attention to the use of poisons, possibly confusing his or her
role with that of a guild member, and so trespassing.
The upshot of this is to consult your DM with respect to the permissible
usage of poisons. Keep in mind the principal reason for restriction of the
use of poison -- the game must offer challenge. If poison is limited or
specially treated, you will understand and co-operate.

Pyremius (god of poison)
Talona (goddess of poisoning)