Opioids are available in many different medicinal forms. Let’s consider this in terms of the route of administration.
Opioids should be prescribed orally wherever possible. This is because the parenteral route offers no analgesic benefit in terms of time to onset or duration of action and oral dosing is generally safer.
There are two main categories of oral opioids, based on their duration of action. Immediate release (IR) oral opioids have a minimum onset of action of 30 minutes and can last up to 4 hours. They are available as liquids (oral solution) and in solid form (tablets and capsules). Modified release (MR) oral opioids usually last 12 hours. Some 24 hourly preparations of morphine are also available. MR opioids are almost always prescribed as tablets or capsules. MR morphine granules are also available, which are mixed with water to form a suspension and are an option for patients with swallowing difficulties and feeding tubes.
Opioid injections are required when the patient is vomiting, cannot swallow or has reduced intestinal absorption (for instance in bowel obstruction). In these scenarios opioids are generally administered subcutaneously. The intravenous and intramuscular routes are more invasive and offer no additional benefit in terms of pain relief.
Injections have the same duration of action as IR oral opioids but a faster onset of action. Sustained, background analgesia (analogous to MR oral opioids) is achieved by delivering the injection via a continuous subcutaneous infusion (CSCI).
Methadone is an opioid sometimes prescribed for severe pain. Its effect varies considerably between individuals and it should only be prescribed under specialist supervision.
Fentanyl and buprenorphine are available as transdermal patches. They must never be prescribed for a patient whose pain is not already stable on an MR oral opioid or CSCI. This is because it takes a long time for these drugs to reach steady state via dermal delivery.
The dose of fentanyl and buprenorphine in a patch is expressed as micro grams per hour. To calculate the amount of opioid in 24 hours you must multiply this figure by 24.
Other formulations are available through different routes of administration. Their duration of action is short, ranging from up to 4 hours for suppositories to minutes for transmucosal fentanyl products (nasal sprays and oro-mucosal formulations). It is recommended that these preparations are prescribed following specialist advice.
In the next step we will look at starting a strong opioid.
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