Final answer:
The statement about categories T36-T65 not requiring an additional external cause code for poisonings, toxic effects, adverse effects, and underdosing is true according to the ICD-10-CM guidelines.
Step-by-step explanation:
The statement is true. According to the ICD-10-CM guidelines, for the categories T36-T65, no additional external cause code is required for poisonings, toxic effects, adverse effects, and underdosing codes. These codes are sufficient for reporting purposes as they already indicate the intent and the substance involved, thereby negating the need for an additional external cause code.