Option D.3 – Analgesics
D.3.1 – Describe and explain the different ways that analgesics prevent pain
The pain receptors in the body are essential for triggering a response that protects it from damage. It is part of our body’s defence system. They also send messages to the brain so that the location of an injury, disease, movement or environmental stress is known. Pain is activated by the presence of prostaglandins. These constrict blood vessels, affect the hypothalamus (which regulates body temperature, leading to fever) and increase the permeability of the capillaries, leading to swelling.
Analgesics releive pain by interfering with the pain receptors. Since pain can be debiliating, relief is sometimes essential. There are two types of analgesics, which act differently:
Mild Analgesics – These prevent the production of prostglandins and therefore the symptoms of pain. Essentially, the intercept the pain stimulus at its source (the nerve endings) so that the message is never sent to the brain and spinal cord. Paracetamol acts differently, since it prevent the release of prostglandins in the brain, rather than the origin of the pain. All mild-analgesics are non-narcotics because they do not affect the functioning of the brain.
Strong Analgesics – These bind to the receptor sites in the brain so that the pain impulses cannot be transmitted further, without depressing the central nervous system. In this way, they alter the brain’s perception of pain. These act in the same manner as natural painkillers (endorphins and enkephalins). Their analgesic effect is only temporary, but is more powerful than that of natural painkillers. Since they affect the functioning of the brain, they are called narcotics.
D.3.2 – Describe the use of derivatives of salicyclic acid as mild analgesics and compare the advantages and disadvantages of using aspirin and paracetamol (acetaminophen)
Aspirin
This is a mild analgesic, called acetyl salicyclic acid. The addition of the acetyl group reduces its acidity, which is important because salicyclic acid is a strong acid that is irritating to the stomach and often induces vomiting.
Aspirin is an ester that has many of the same properties of salicyclic acid, in that it gives relief from minor aches and pains, reduces fever (also called antipyretic effect), acts as an anti-inflammatory agent and as an anti-clotting agent.
Paracetamol
This is another mild analgesic, also called acetaminophen. Its structure is very different to aspirin, as it is an amide. Unlike aspirin, it does not have as many side-effects, but also does not reduce inflammation. It is possible to overdose on paracetamol and cause fatal damage, especially if taken with alcohol.
D.3.3 – Compare the structures of morphine, codeine and diamorphine (heroin, a semi-synthetic opiate)
Morphine, codeine and diamorphine are all opiates. They are narcotics, which act as analgesics, giving the user a sense of euphoria, peace, tranquillity, loss of feeling and sometimes paralysis. They can also cause constipation, constriction of the pupil in the eye and suppression of the cough reflex. In terms of strength, they are ordered:
All the compounds have similar structures:
- alkene
- aromatic ring
- ether
- tertiary amine functional groups
The structures above show that each has variation in their functional groups.
Codeine – benzene ring, two ether groups, alkene, one alcohol group and a tertiary amine
Morphine – benzene ring, one ether, alkene, two alcohol groups and a tertiary amine
Heroin – benzene ring, one ether, alkene, two ester-ethanoate groups and a tertiary amine
Heroin is produced in a condensation reaction between morphine and ethanoic acid, forming an ester linkage. It is a semi-synthetic drug because it can be found naturally, but is usually produced in this reaction of morphine:
Heroin is more effective because it is a more lipid-soluble molecule. This allows it to cross the blood-brain barrier more easily.
D.3.4 – Discuss the advantages and disadvantages of using morphine and its derivatives as strong analgesics
Short-Term Effects
Morphine is a much stronger analgesic than any other compound. However, it is also very addictive, and must be injected. Addiction leads to mental fogginess and mood swings.
Heroin is stronger and more addictive than morphine. It is also more dangerous because it depresses the central nervous system, causes respiratory depression, drowsiness and decreases gastrointestinal movement (which in turn causes constipation, nausea and vomiting). Tolerance develops very quickly, so it is not used for medical purposes. Immediate effects include a sense of well-being, dulling of pain, lowered fear and tension.
Long-Term Effects
Heroin use leads to constipation, reduced libido and appetite, and poor nutrition. If addicts try to quit, they will experience unpleasant withdrawal symptoms, such as cold sweats and anxiety. It is also very costly, and is illegal to deal in most countries. Some addicts will use crime to obtain the money for it. There is also higher risk of contracting HIV due to needle-sharing, as heroin must be taken intravenously. Its use is also associated with serious health conditions such as collapsed veins, spontaneous abortion and infectious diseases.
On the other hand, whilst codeine is only 10% as effective, it is not addictive, which makes it safer than morphine. In some places, it is available over the counter.