Final answer:
A drug that binds to a receptor and produces a similar effect to the body's natural substance is called an agonist. These drugs are often used in treatments to mimic endogenous neurotransmitters, like in Parkinson's disease therapy where dopamine agonists are utilized.
Step-by-step explanation:
A drug that has an affinity for a receptor and produces an effect when it combines with this receptor is termed an agonist. An agonist is any exogenous substance that binds to a receptor and produces a similar effect to the endogenous ligand. This is in contrast to an antagonist, which binds to a receptor and produces an opposing effect to the endogenous ligand. Agonists are commonly used in medical treatments to increase the activity of a neurotransmitter system. For example, dopamine agonists may be prescribed in conditions such as Parkinson's disease to mimic the effects of dopamine and bind to dopamine receptors, thus compensating for the low levels of the neurotransmitter.