An important property of painkillers is the time they stay in the body. How long after administration will they have an effect? The longer, the better. The dose of the drug may vary from less than 10 mg (a pinhead-sized portion) do 1000 mg (Alka Seltzer tablet size). Part of the reason for this difference is time, what the body needs, to get rid of the drug treated like a foreign substance (even though it brings relief). Sometimes days may pass, before the last drug molecule is removed from the body, even though it has long stopped working. Therefore, the time of complete disappearance of the substance turns out to be not very helpful parameter in determining the dose amount.
Scientists use a parameter to compare the duration of drugs in the body, called the half-life of the drug. It is time, by which the body removes half of the entered dose. If for some drug the half-life is 10 hours, means, that after ten hours, half of the original dose will remain in the body. After another ten hours, its quantity will again be halved (that is, up to a quarter of the original value). Even after 100 hours, we will be able to detect traces of the drug, the number of which will be equal to a thousandth of a part (0,1 %) original dose.
Even when we are talking about such a complex system, what are the human body and the chemical processes taking place in it, the half-life concept is still useful, although this figure may vary from person to person. For example, it depends, for example, on the efficiency of the liver and kidneys. The data presented in the table below show the average half-lives of some substances, healthy body.
Half-life of substances in the human body.
Lek – Half-life
aspirin – 15 minutes *
caffeine – 5 hours
cocaine – 45 minutes
valium – 18 hours
morphine – 3 hours
nicotine – 2 hours
streptomycyna – 5 hours
* Fifteen minutes seems too short a time, but aspirin works by converting into other substances with a much longer half-life.
About half of the chemicals tested are carcinogenic, which doesn't mean, that they will automatically cause cancer. Each serving of food contains a certain amount of arsenic, which is a known carcinogen. However, our bodies are able to cope with small doses, and, in fact, they may even need them. To, that the substance causes cancer in animals, to whom it is served in large amounts, does not mean, that small doses of it cause cancer in humans. Before we conclude, whether the substance could cause cancer in laboratory mice, we have to destroy the poor animal's natural defenses for days with large doses of the substance. If as a result of the test it turns out, that the test chemical contributed to the development of cancer in mice, it is in the United States that this substance is not approved for use by the American Food and Drug Administration (FDA), an organization recognized as a global authority in the field of security.