Final answer:
The development of a headache after administering nitroglycerin to a patient is classified as a side effect of the medication.
Step-by-step explanation:
When patients are given nitroglycerin and they develop a headache as a result, this would be considered a side effect. Side effects are secondary, typically undesirable effects of a drug or medical treatment, beyond the intended therapeutic effects. In the case of nitroglycerin which is used to treat angina and other heart conditions by dilating blood vessels and improving blood flow, headaches occur due to the dilation of blood vessels in the brain, similar to what happens in migraines. It's important for healthcare providers to be aware of the potential side effects of medications to manage and inform patients accordingly.