Final answer:
A drug is contraindicated for a patient when it may cause harm or has no positive effect. Contraindications are factors that make it unsafe or inappropriate to use a particular drug for an individual.
Step-by-step explanation:
A drug is contraindicated for a patient when it may cause harm or has no positive effect. Contraindications are factors that make it unsafe or inappropriate to use a particular drug for an individual. These can include allergies, interactions with other medications, pre-existing medical conditions, or physiological factors that could lead to adverse reactions or ineffective treatment. For example, if a patient has a known allergy to a drug or if the drug has been shown to have no therapeutic benefit for the patient's condition, it would be contraindicated.