One of the most common ways of signing for "poison" is to do the sign
BONES in context.
You can make the concept even more obvious by using a negative facial expression
but this isn't necessary if the context makes your concept clear.

There is another fairly common sign for "poison" that twists a "P" handshape on
the neck -- pivoting a couple of times on the tip of the middle finger. While some people
may call this sign a "Signed English" type sign -- it is still used by quite a
few people in the Deaf Community and worth knowing. Additionally, it is a
low context dependent sign -- meaning that it doesn't need a lot of context to
be understood (compared to the BONES sign which does indeed need a lot of
context to be understood without ambiguity).
POISON (version 2: initialized with a "P" handshape)

POISON (version 2: initialized)
Other versions:
poison-[MEDICINE+context+negative-facial-expression]
The sign "MEDICINE" can be (and is on occasion) used to mean "poison."
Typically such use will be in high context situations, use full or partial
mouthing, use an appropriate facial expression, and will be part of a common
phrase associated with the meaning of "poisoning" such as "food poisoning."
For example, you could sign "FOOD-MEDICINE" in context to create the meaning of
"food poisoning."
However, just be cause you "can" sign something doesn't mean you should.
Check with your local Deaf Community to see which version of POISON is common in
your area.
poison-[MEDICINE+context+inflection] is legitimate version worthy of
documentation and addition to the receptive vocabulary of ASL interpreters and
educators -- as well as adoption for general usage in some regions where it may
be a dominant version.
For example some people sign "food poisoning" by signing FOOD
MEDICINE-[negative-facial-expression]
Food poisoning [FOOD MEDICINE version]
Note that other people may choose to sign FOOD BONES (in context) to mean "food poisoning." Or even "FOOD POISON-[initialized version]" So it is a good idea to stay flexible regarding the signing of "food poisoning."
Another way to sign "poison" is to cross your index fingers in an "X" shape.
This version isn't used as much in ASL as the other versions. This version
would also require a lot of context. I wouldn't recommend using this version but
I'm sharing it here with you so that you can recognize it if you see it.
However the "crossed index fingers" sign can also (with a neutral facial
expression) be used to mean "X out of that window" meaning to click on the "X"
in the corner of a screen window to close the window. Also compare this
sign versus the sign for CONFLICT.
POISON [crossed-index-fingers version / "X" version]

POISON [crossed-index-fingers version / "X" version]
Notes:
Question: A student asks: "What is the sign for "poison oak" ? I
have poison oak near where I live and would like to know the sign. I'm having a
hard time finding the sign."
Response:
For "poison oak" do the crossbones version of the sign for poison and then
fingerspell "oak."
There are other signs for poison but the crossbones version is a classic.
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