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Exposure to a particular drug over a prolonged period of time can often lead to the development of an insensitivity to that drug. If a drug is being taken for a particular medical purpose (eg. the relief of pain), one might find that after a while the same pain relief can only be obtained by increasing the dose. This is known as ‘tolerance.’
Some drugs are particularly prone to this effect. The body’s response to opiates like morphine or pethidine is an example. After taking morphine for even a short time, the effect of a particular dose will lessen and greater amounts will be needed to achieve the same pain-relieving effect.
After a period off the drugs, one’s sensitivity usually returns. This explains why people often accidentally die when taking illegal narcotics like heroin. A person who regularly uses heroin soon develops a tolerance for it. If they are unable to continue taking the drug - perhaps because their supply has broken down or perhaps they have spent time in an institution, they will redevelop their sensitivity. When a new supply becomes available, their greater sensitivity increases the likelihood of accidental death (see Chapter 10 for more information on the opiates).
Tolerance to a particular drug can be an important factor when choosing a drug to end one’s life. If a seriously ill person has been taking a drug for some time and has developed a tolerance for this particular drug, the necessary ‘lethal dose’ for the drug can be higher than that usually quoted.