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The science of drug processing by the body is called pharmacokinetics. This science helps prescribers to choose the correct dosage, route of administration, treatment frequency and duration, as these can vary considerably between individuals. Clinical pharmacokinetics is used in clinical trials, where drugs are tested on people. From blood or urine analysis, pharmacokinetics equations allow researchers to know how long it takes to the body to eliminate the drug, where the drug goes in the body (in the bloodstream or elsewhere), and what the average right dosage is.

For more on pharmacokinetics:
The pharmacology education project provides useful explanations on the principles of pharmacokinetics and pharmacology. The interactive clinical pharmacology website uses well-designed interactive schemes to explain the core concepts of pharmacokinetics.The NIH’s webpage “Medicines by design” provides an overview of the pharmacology concepts and drug research.
For every drug, the amount given to a patient has to be right, or within the “therapeutic window.” If the dose is too low, the drug will have no effect, while if it is too high, the drug may cause toxic effects. These toxic effects can range from side effects such as fatigue or nausea, to fatal injuries, such as liver failure in paracetamol overdose. The FDA publishes medication guides and useful information by drug class to dig deeper – for instance on a specific pain killers class, the NSAIDS, on the risks, benefits, and mechanisms of action for different drugs.

Adjusting drug doses is particularly difficult in children, as drug trials are generally conducted on adults and children’s physiology varies considerably with age - especially their liver enzyme efficiency to process drugs. Children’s liver enzymes are immature before 3 months old. For any drug, similar doses per weight as an adult would result in far more drug in the bloodstream for a much longer time in young children. This can lead to dangerous effects. Liver enzyme activity quickly increases until the age of 3 years old, reaching 2 to 6 times adult values. Drug doses per weight need to be higher in this age range, as the drugs are better and faster processed by the children’s body. From 3 to 10 years old, liver enzyme activity gradually decreases to reach values similar to adults. These variations make it difficult to adjust doses. (some references: Johnson T.N., 2003. Toxicology 192, 37-48; Pons G. et al. 1988. Pediatric Research 23, 632-636). The NIH website on medicines and children provide basic information and useful links.

Many parameters can influence the efficiency and speed of drug processing by the body, including age, genetics, diet, disease, and pregnancy. The elderly tend not to eliminate drugs efficiently, due to a decrease in liver and/or kidney effectiveness. For more information on senior health, visit here.

The body's efficiency when processing drugs may change during pregnancy. Some medicines may harm the baby or the mother during pregnancy. The US national library of medicine give general information on medication use during pregnancy. The FDA regularly reviews the risks and benefits of medication during pregnancy for different drug classes, for instance, pain medication.