Many drugs work by mimicking a naturally occurring hormone or neurotransmitter in the body.
If the drug causes the receptor to respond in the same way as the naturally occurring substance, then the drug is referred to as an agonist. Examples of agonists are nicotine and morphine. These drugs bind to the receptor, but do not produce a response. Because the drug prevents the receptor from binding to the normal hormone or neurotransmitter, it has an inhibitory effect on the naturally occurring substance.