Read the office coding guidelines, you will find the answers. I copied and pasted some here for you.
Adverse Effects, Poisoning, Underdosing and Toxic Effects Codes in categories T36-T65 are combination codes that include the substance that was taken as well as the intent. No additional external cause code is required for poisonings, toxic effects, adverse effects and underdosing codes.
1) Do not code directly from the Table of Drugs Do not code directly from the Table of Drugs and Chemicals. Always refer back to the Tabular List.
2) Use as many codes as necessary to describe Use as many codes as necessary to describe completely all drugs, medicinal or biological substances.
3) If the same code would describe the causative agent If the same code would describe the causative agent for more than one adverse reaction, poisoning, toxic effect or underdosing, assign the code only once.
4) If two or more drugs, medicinal or biological substances If two or more drugs, medicinal or biological substances are taken, code each individually unless a combination code is listed in the Table of Drugs and Chemicals. If multiple unspecified drugs, medicinal or biological substances were taken, assign the appropriate code from subcategory T50.91, Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances.
5) The occurrence of drug toxicity is classified in ICD-10- CM as follows:
(a) Adverse Effect When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug (T36-T50). The code for the drug should have a 5th or 6th character “5” (for example T36.0X5-) Examples of the nature of an adverse effect are tachycardia, delirium, gastrointestinal hemorrhaging, vomiting, hypokalemia, hepatitis, renal failure, or respiratory failure.
(b) Poisoning When coding a poisoning or reaction to the improper use of a medication (e.g., overdose, wrong substance given or taken in error, wrong route of administration), first assign the appropriate code from categories T36-T50. The poisoning codes have an associated intent as their 5th or 6th character (accidental, intentional self-harm, assault and undetermined. If the intent of the poisoning is unknown or unspecified, code the intent as accidental intent. The undetermined intent is only for use if the documentation in the record specifies that the intent cannot be determined. Use additional code(s) for all manifestations of poisonings. If there is also a diagnosis of abuse or dependence of the substance, the abuse or dependence is assigned as an additional code.
Examples of poisoning include:
(i) Error was made in drug prescription Errors made in drug prescription or in the administration of the drug by provider, nurse, patient, or other person.
(ii) Overdose of a drug intentionally taken If an overdose of a drug was intentionally taken or administered and resulted in drug toxicity, it would be coded as a poisoning.
(iii) Nonprescribed drug taken with correctly prescribed and properly administered drug If a nonprescribed drug or medicinal agent was taken in combination with a correctly prescribed and properly administered drug, any drug toxicity or other reaction resulting from the interaction of the two drugs would be classified as a poisoning.
(iv)Interaction of drug(s) and alcohol When a reaction results from the interaction of a drug(s) and alcohol, this would be classified as poisoning. See Section I.C.4. if poisoning is the result of insulin pump malfunctions.
(c) Underdosing Underdosing refers to taking less of a medication than is prescribed by a provider or a manufacturer’s instruction. Discontinuing the use of a prescribed medication on the patient's own initiative (not directed by the patient's provider) is also classified as an underdosing. For underdosing, assign the code from categories T36-T50 (fifth or sixth character “6”).
Codes for underdosing should never be assigned as principal or first-listed codes. If a patient has a relapse or exacerbation of the medical condition for which the drug is prescribed because of the reduction in dose, then the medical condition itself should be coded. Noncompliance (Z91.12-, Z91.13- and Z91.14-) or complication of care (Y63.6-Y63.9) codes are to be used with an underdosing code to indicate intent, if known. (d) Toxic Effects When a harmful substance is ingested or comes in contact with a person, this is classified as a toxic effect. The toxic effect codes are in categories T51-T65. Toxic effect codes have an associated intent: accidental, intentional self-harm, assault and undetermined.