Following on from the previous post on antibiotics, it seemed logical to also take a look at the drugs we take to relieve pain. Painkilling drugs, or analgesics, come in a number of forms, but fall broadly into two main classes: non-steroid anti-inflammatory drugs (NSAIDs) and opioids. This graphic takes a look at a selection of common painkillers, their common brand names, and how they work. The mechanism of action is in many cases not fully understood, but we have a broad idea of how the two classes exert their effects.
Non-steroid anti-inflammatory drugs include analgesics such as aspirin and ibuprofen, shown in the graphic, as well as naproxen. These drugs all work by inhibiting the synthesis of a class of chemical compounds called prostaglandins. Prostaglandins are produced by the body at the sites of tissue damage or infection, and, along with other chemicals produced by the body in these cases, they contribute significantly towards inflammation and pain. NSAIDs work by inhibiting the activity of two enzymes, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). These enzymes catalyse prostaglandin synthesis, so when their activity is inhibited, so is the body’s manufacture of prostaglandins. The net result is a reduction in inflammation, and subsequently a reduction in pain.
Painkillers have no innate method of reaching only the site of pain, but rather are distributed evenly through-out the body when you take them. They’re also non-discriminative in terms of their action, and will inhibit prostaglandin synthesis all over your body, not just at the site of pain. Prostaglandins don’t just have a role in pain in our bodies – they’re also found in the gut, where their role includes protection of the gut lining. As such, use of NSAIDs increases the risk of stomach ulcers and gastrointestinal irritation. For this reason, during sustained course of NSAIDs (for example, after surgery), drugs to help protect the gut lining may also be prescribed.
The second major class of painkillers are the opioids. These are a class of drugs related to morphine, the compound found in significant concentrations in the opium poppy. The opium poppy itself has been used for its natural painkilling properties for centuries, as the opium which can be extracted from it contains around 12% morphine. The synthetic drug heroin is also obtained from a simple chemical modification of morphines structure; many of the painkilling opioids have similarly minor differences in chemical structure.
The opioids work in a different way to the NSAIDs; rather than combatting the pain at its source, they instead prevent the sensation of pain by binding to and blocking the receptors in the brain and spinal cord that are responsible for the transmission of the sensation of pain. This group of receptors are known as opioid receptors, and there are four different subtypes; the exact manner in which many of the opioids inhibit pain by binding to these receptors isn’t fully understood.
Opioids may be potent painkillers, but their overuse comes with the spectre of opioid addiction. Excessive use leads to over-stimulation of the brain’s ‘reward’ pathways; the brain also tries to compensate by reducing the number of opioid receptors, meaning progressively more of the opioid is required to achieve the same highs. No single treatment is effective for all opioid dependent patients, and oddly enough, some of the opioids drugs are used in some treatments. Methadone and buprenorphine are both commonly prescribed, as they are longer acting than, for example, heroin, and so allow for less frequent dosing. Tolerance is also slow to develop, and as such their use has been associated with a reduction in the use of other opioids in opioid dependent individuals.
Although fentanyl is the most potent opioid shown on the chart, more powerful opioids do exist. The most potent used in humans is sufentanil, considered to be approximately 500-1000 times stronger than morphine. Carfentanil, with a potency considered to be around 10,000 times that of morphine, is used as a general anaesthetic in large animals.
Paracetamol is something of an oddity amongst the painkillers, in that it’s categorised separately, rather than in one of the two main groups. Part of the reason behind this is that we still don’t have a very good idea of how paracetamol exerts its painkilling effects. It’s thought that, like the NSAIDs, it works by inhibiting cyclooxygenase enzymes, but there are also suggestions that it works on the endocannabinoid system in the body, which plays a part in pain. In short, we still don’t really know how it functions – a great summary of some of the different theories is put forward in a video here by the American Chemical Society’s Reactions team.
Paracetamol also has a detraction, in that its toxic dose is relatively close to the effective dose. Excessive use or overdose can lead to damage to cells in the liver, which can in turn lead to liver failure and death. This is the reason why, when you go to the supermarket, there’s a limit on the number of boxes of paracetamol you can buy at once!
References & Further Reading