Any variables that can be used to distinguish poisoning from injury in NHIS Injury data?

When conducting the interview, the NHIS simply asked “Did you have injury OR poisoining…”. Unfortunately, no original NHIS variables were designed to indicate which incident was an injury or poisoning.

Has the IHIS consctructed such a variable (using for example the ICD-9 code of the cause of an injury)?

Prior to 2000, poisonings were asked about separately from injuries and information on the frequency of poisoning can be found in the person-level variables POISON3MO and POISON3MONO. Beginning in 2000, as you noted, poisonings were asked about in conjunction with injuries. You can find the number of times an injury was caused by poisoning at the person level using the variable INJCAUSPOIN.
For more information on injury and poison data, please see this User Note.
Hope this helps.