PRN drugs given 'as and when needed' can cause withdrawal symptoms leading to more drugs being prescribed than is safe for young or elderly people.
PRN: Pro re nata is commonly used in medicine to mean "as needed" or "as the situation arises." It is generally used as the acronym PRN to refer to dosage of prescribed medication that is not scheduled; instead administration is left to the caregiver or the patient's prerogative. PRN administration of medication is not meant to imply and should never allow for exceeding a prescribed daily regimen. http://en.wikipedia.org/wiki/Pro_re_nata
How Medicines Work
both pharmacokinetics and pharmacodynamics is the study of how a medicine acts in the body.
Pharmacokinetics is the study of the processes and rate at which a medicine passes through the body.
Absorption is the process by which a medicine enters the blood stream.
Distribution refers to the transportation of a medicine to the site of action.
Metabolism is the process whereby a medicineís structure and properties are altered,
Genetic variation may influence all of these processes, since they involve numerous different molecules produced by genes, such as transport proteins and pumps, carriers and enzymes.
Research in pharmacogenetics has traditionally focused on individual variation in the metabolism of medicines. The process of metabolism generally takes place in the liver where medicines are acted upon by enzymes.
Variation in the rate of metabolism of a medicine by an enzyme can substantially alter how a person responds to that medicine. For example, rapid metabolism of a medicine can cause it to be ineffective, and slow or nonmetabolism can lead to the accumulation of toxic amounts of the medicine in the body.
Variation in proteins that metabolise medicines often affects response to more than one medicine.
Excretion is the removal of the medicine from the body through the kidneys and liver.
Pharmacodynamics is the study of how a medicine works in the body. Most medicines work by interacting with the control systems of the body such as receptors, carrier molecules or enzymes.
An individualís reaction to a particular medicine is therefore affected by genetic variation in these molecules.
Historically, the development of medicines has proceeded on the presumption that these molecules are genetically homogeneous in the patient population. However, many studies in recent years have shown that this is not necessarily the case.
Some abbreviated terms
APRIL Adverse Psychiatric Reactions Information Link
ADR's adverse drug reactions
SAE's serious adverse events that occur during clinical trials often undisclosed
MHRA Medicines and Healthcare Products Regulatory Agency http://www.mhra.gov.uk/index.htm
FDA US Food and Drug Administration http://www.fda.gov/medwatch/safety/2009/safety09.htm
West Midlands ADR centre change of web address http://adr.org.uk/
ABPI Association of British Pharmaceutical Industries www.medicines.org.uk