An opioid is a chemical substance that has a morphine-like action in the body. The main use is for pain relief. These agents work by binding to opioid receptors, which are found principally in the central nervous system and the gastrointestinal tract. The receptors in these two organ systems mediate both the beneficial effects, and the undesirable side effects.

Weak Opioids includes Codeine, Dextropropoxyphene, Dihydrocodeine, Tramadol, Partial agonists of morphine and used for musculo-skeletal and visceral pain, usually in combination with paracetamol and/or NSAIDs.

Codeine is in a group of drugs called narcotic pain medicines.Codeine is used to treat mild to moderate pain. Codeine may also be used for purposes other than those listed in this medication guide.

Codeine is a weak opioid (morphine like effect) which has no analgesic effect of its own. It is a pro-drug with 10% of the oral dose is converted to morphine by the liver. About 7% of the population do not have the necessary enzyme to convert it, and therefore codeine is ineffective in these people. Studies suggest that it most effective when taken with paracetamol.

Codeine side effects hives; difficulty breathing; swelling of your face, lips, tongue, or throat. shallow breathing, slow heartbeat; feeling light-headed, fainting; confusion, agitation, hallucinations, unusual thoughts or behavior seizure (convulsions); problems with urination.

Dextropropoxyphene is a weak opioid which has mixed activity at morphine receptors. Dextropropoxyphene is an analgesic in the opioid category. It is used to treat mild to moderate pain and as an antitussive. It can be used to ease pain before, during and after an operation. It is often combined with acetominophen in the preparation co-proxamol (Darvocet in the US and CAPADEX in AUS).

At low doses Dextropropoxyphene is active at the receptors producing analgesia. At higher doses it inhibits these receptors producing a ceiling effect on analgesia. Dextropropoxyphene is therefore a mixed agonist-antagonist. When used with other morphine-like drugs, mixed agonist-antagonist drugs reduce the overall analgesia rather than producing an additive effect.  Darvocet overdose is commonly broken into two categories: liver toxicity (from acetaminophen poisoning) and dextropropoxyphene overdose. Many users experience toxic effects from the acetaminophen in pursuit of the endlessly-increasing dose required to achieve euphoria. They suffer acute liver toxicity, which causes severe stomach pain, nausea, and vomiting (all of which are increased by light or stimulation of the sense of sight).

Dihydrocodeine is a weak opioid but slightly stronger than codeine. Still needs to be converted to morphine in the liver, Same side effects as codeine.

Tramadol is a weak opioid which is also converted to another active compound by the liver. It has two modes of action 1) a weak morphine like effect 2) stimulation of the descending nerves from the brain which inhibit the dorsal horn of the spinal cord, by preventing reuptake of noradrenaline and 5-HT (serotonin). Please check Tramadol for more information.

Partial agonists of morphine - these drugs are like dextropropoxyphene in that they have mixed actions at morphine receptors, and have a ceiling effect in higher dosage. Examples are pentazocine, butorphanol, and nalbuphine. They are weak in their analgesic action, but unlike morphine do not cause respiratory depression. Side effects like dysphoria and nausea are common. Pentazocine has an addiction potential and is slowly being withdraw from clinical use. Nalbuphine is used by paramedics prior to transfer to hospital