Drugs that mimic the natural messenger by switching on the receptor, these are called agonists. These are useful when there is lack of natural chemical messenger.

The correct answer is option 'C': Agonists.

Explanation:
- Drugs that mimic the natural messenger by switching on the receptor are called agonists.
- Agonists are substances that bind to and activate specific receptors in the body, producing a physiological response.
- They act by mimicking the natural messenger or ligand that would normally bind to the receptor and initiate a response.
- When an agonist binds to the receptor, it activates the receptor and triggers a series of biochemical events within the cell, leading to a specific physiological response.
- Agonists can be either endogenous (naturally occurring within the body) or exogenous (introduced from outside the body).
- Examples of endogenous agonists include neurotransmitters like dopamine, serotonin, and acetylcholine, which bind to and activate specific receptors in the brain and nervous system.
- Exogenous agonists include drugs such as morphine, which binds to opioid receptors in the brain, and adrenaline, which activates adrenergic receptors in the body.
- Agonists can have different levels of activity, ranging from full agonists that produce a maximal response to partial agonists that produce a submaximal response.
- The efficacy of an agonist refers to its ability to activate the receptor and produce a response, while potency refers to the concentration or dose required to produce a particular response.
- Agonists are commonly used in medicine to treat various conditions. For example, agonists that mimic the action of neurotransmitters can be used to treat Parkinson's disease (e.g., levodopa), while agonists that activate adrenergic receptors can be used to treat asthma (e.g., albuterol).
- It is important to note that agonists can also have side effects and potential risks, and their use should always be guided by a healthcare professional.