In choosing analgesics, the severity and response to other medication determines the choice of agent; the WHO pain ladder, originally developed in cancer-related pain, is widely applied to find suitable drugs in a stepwise manner. The analgesic choice is also determined by the type of pain: for neuropathic pain, traditional analgesics are less effective, and there is often benefit from classes of drugs that are not normally considered analgesics, such as tricyclic antidepressants and anticonvulsants.
The major classes
Paracetamol and NSAIDs
The exact mechanism of action of paracetamol/acetaminophen is uncertain, but it appears to be acting centrally (in the brain rather than in nerve endings). Aspirin and the other non-steroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenases, leading to a decrease in prostaglandin production. This reduces pain and also inflammation (in contrast to paracetamol and the opioids).
Paracetamol has few side effects and is regarded as safe, although intake above the recommended dose can lead to liver damage, which can be severe and life-threatening, and occasionally kidney damage. NSAIDs predispose to peptic ulcers, renal failure, allergic reactions, and occasionally hearing loss, and they can increase the risk of hemorrhage by affecting platelet function. The use of aspirin in children under 16 suffering from viral illness has been linked to Reye's syndrome, a rare but severe liver disorder.
These drugs have been derived from NSAIDs. The cyclooxygenase enzyme inhibited by NSAIDs was discovered to have at least 2 different versions: COX1 and COX2. Research suggested that most of the adverse effects of NSAIDs were mediated by blocking the COX1 (constitutive) enzyme, with the analgesic effects being mediated by the COX2 (inducible) enzyme. The COX2 inhibitors were thus developed to inhibit only the COX2 enzyme (traditional NSAIDs block both versions in general). These drugs (such as rofecoxib and celecoxib) are equally effective analgesics when compared with NSAIDs, but cause less gastrointestinal hemorrhage in particular. However, post-launch data indicated increased risk of cardiac and cerebrovascular events with these drugs; this is probably due to an imbalance in blood coagulation. Rofecoxib (marketed as Vioxx) was subsequently withdrawn from the market. The role for the remaining members of this class of drug is debated.
Opiates and morphinomimetics
Morphine, the archetypal opioid, and various other substances (e.g. codeine, oxycodone, hydrocodone, dihydromorphine, pethidine) all exert a similar influence on the cerebral opioid receptor system. Tramadol and buprenorphine are thought to be partial agonists of the opioid receptors. Tramadol is structurally closer to venlafaxine than to codeine and delivers analgesia by not only delivering "opiate-like" effects (through mild agonism of the mu receptor) but also by acting as a weak but fast-acting serotonin and norepinephrine reuptake inhibitor. Dosing of all opioids may be limited by opioid toxicity (confusion, respiratory depression, myoclonic jerks and pinpoint pupils), seizures (tramadol), but there is no dose ceiling in patients who tolerate this.
Opioids, while very effective analgesics, may have some unpleasant side-effects. Up to 1 in 3 patients starting morphine may experience nausea and vomiting (generally relieved by a short course of antiemetics). Pruritus (itching) may require switching to a different opioid. Constipation occurs in almost all patients on opioids, and laxatives (lactulose, macrogol-containing or co-danthramer) are typically co-prescribed.
When used appropriately, opioids and similar narcotic analgesics are otherwise safe and effective, however risks such as addiction and the body becoming used to the drug (tolerance) can occur. The effect of tolerance means that drug dosing may have to be increased if for a chronic disease.
Flupirtine is a centrally acting K+ channel opener with weak NMDA antagonist properties. It is used in Europe for moderate to strong pain and migraine and its muscle relaxant properties. It has no anticholinergic properties and is believed be devoid of any activity on dopamine, serotonin or histamin receptors. It is not addictive and tolerance does not develop.3
In patients with chronic or neuropathic pain, various other substances may have analgesic properties. Tricyclic antidepressants, especially amitriptyline, have been shown to improve pain in what appears to be a central manner. Nefopam is used in Europe for pain relief with concurrent opioids. The exact mechanism of carbamazepine, gabapentin and pregabalin is similarly unclear, but these anticonvulsants are used to treat neuropathic pain with differing degrees of success. Anticonvulsants are most commonly used for neuropathic pain as their mechanism of action tends to decrease the firing of specific nerve systems.
Specific forms and uses
Analgesics are frequently used in combination, such as the paracetamol and codeine preparations found in many non-prescription pain relievers. They can also be found in combination with vasoconstrictor drugs such as pseudoephedrine for sinus-related preparations, or with antihistamine drugs for allergy sufferers.
While the use of paracetamol, as well as aspirin, ibuprofen, naproxen, and other NSAIDS concurrently with weak to mid-range opiates (up to about the hydrocodone level) has been said to show beneficial synergistic effects by combatting pain at multiple sites of action, several combination analgesic products have been shown to have little efficacy benefits when compared to similar doses of their individual components. These combination analgesics can often result in significant adverse events including accidental overdoses, most often due to confusion which arises from the multiple (and often non-acting) components of these combinations.
Topical or systemic
Topical analgesia is generally recommended to avoid systemic side-effects. Painful joints, for example, may be treated with an ibuprofen- or diclofenac-containing gel; capsaicin also is used topically. Lidocaine, an anesthetic, and steroids may be injected into painful joints for longer-term pain relief. Lidocaine is also used for painful mouth sores and to numb areas for dental work and minor medical procedures.
Tetrahydrocannabinol (THC) and some other cannabinoids, either from the Cannabis sativa plant or synthetic, have analgesic properties, although the use of cannabis derivatives is currently illegal in many countries. A recent study finds that inhaled cannabis is effective in alleviating neuropathy and pain resulting from e.g. spinal injury and multiple sclerosis.6 Other psychotropic analgesic agents include ketamine (an NMDA receptor antagonist), clonidine and other a2-adrenoreceptor agonists, and mexiletine and other local anaesthetic analogues.Atypical and/or adjuvant analgesics
Orphenadrine, cyclobenzaprine, scopolamine, atropine, gabapentin, first-generation antidepressants and other drugs possessing anticholinergic and/or antispasmodic properties are used in many cases along with analgesics to potentiate centrally acting analgesics such as opioids when used against pain especially of neuropathic origin and to modulate the effects of many other types of analgesics by action in the parasympathetic nervous system. Dextromethorphan has been noted to slow the development of tolerance to opioids and exert additional analgesia by acting upon the NMDA receptors; some analgesics such as methadone and ketobemidone and perhaps piritramide have intrinsic NMDA action. High-alcohol liquor has been used in the past as an agent for dulling pain, due to the CNS depressant effects of ethyl alcohol, a notable example being the American Civil War. However, the ability of alcohol to "kill pain" may be inferior to many analgesics used today (e.g. morphine, codeine). As such, the idea of alcohol for analgesia is generally considered a primitive practice in virtually all industrialized countries today.
The use of adjuvant analgesics is an important and growing part of the pain-control field and new discoveries are made practically every year. Many of these drugs combat the side effects of opioid analgesics, an added bonus. For example, antihistamines including orphenadrine combat the release of histamine caused by many opioids, methylphenidate, caffeine, ephedrine, dextroamphetamine, and cocaine work against heavy sedation and may elevate mood in distressed patients as do the antidepressants. A well-accepted benefit of THC to chronic pain patients on opioids is its superior anti-nauseant action. Some think it would make more sense to use the synthetic THC capsule (trade name Marinol), which is administered orally. However, in patients suffering from nausea, the swallowing of the capsule itself may provoke vomiting. Likewise, the use of medicinal cannabis remains a debated issue.