The opioid analgesics discussed in this section are "mu-agonist" drugs.

• The term "agonist" refers to a drug that binds to pain receptors to produce analgesia.
• Opioid analgesics include morphine and other drugs that relieve pain by binding to mu receptors in the nervous system.
• The term "mu agonist" is used interchangeably with the terms full agonist, pure agonist and morphine-like drugs.
  

Examples of opioid analgesics are:

Codeine
Oxycodone
Meperidine (Demerol)
Propoxyphene (Darvon)
Fentanyl
Hydrocodone
Tramadol (Ultram)
Morphine
Hydromorphone (Dilaudid)
Methadone
Levorphanol (Levo-Dromoran)
Oxymorphone (Numorphan)

Of the three groups of analgesics – opioids, nonopioids, and coanalgesic drugs, only the morphine-like opioids have no analgesic ceiling. In other words, higher doses increase analgesia and only adverse effects limit how high the dose can be. Thus, there is no set maximum dose for morphine and morphine-like opioids. If a dose of morphine fails to relieve pain and the patient experiences no adverse effects, the dose may be increased by 25% to 50%. Some morphine-like opioids are more effective than others for taking advantage of the no ceiling effect. Those drugs most appropriate for providing increasing pain relief for rapidly escalating severe pain include morphine, hydromorphone, and fentanyl. To treat severe escalating pain, the drugs should be given intravenously for rapid onset of analgesia and then titrated to effect. Titrating to effect refers to the practice of continuing to increase the dose of the opioid until the patient obtains a desired level of pain relief or until adverse reactions are unmanageable and intolerable.

Morphine remains the standard against which all other opioid drugs are compared. Extensive clinical experience and multiple routes of delivery, including controlled release, make it the opioid most commonly used for cancer pain.

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