A drug is defined as a pure chemical compound with a specific structure that influences both its chemical and biological properties. The effects of drugs occur at the cellular level, and they typically produce multiple responses, including side effects, which are effects outside of the intended therapeutic outcomes. The primary purpose of a drug, known as the therapeutic effect, aligns with its clinical indication, which specifies the conditions it's intended to treat. For instance, beta blockers serve the therapeutic indication of treating hypertension by reducing heart rate. Furthermore, contraindications exist, indicating situations where a drug should not be used due to potential adverse interactions with a patient's other medical conditions.