Editing Drug interaction (section)

== Underlying factors ==
The factors or conditions that predispose the appearance of interactions include factors such as [[old age]].<ref name="Baños">{{cite book |author=Baños Díez, J. E. |url=https://books.google.com/books?id=gsb6J2sYdisC |title=Farmacología ocular |author2=March Pujol, M |publisher=Edicions UPC |year=2002 |isbn=978-8483016473 |edition=2da |pages=87 |language=es |access-date=23 May 2009}}</ref> This is where human physiology changing with age may affect the interaction of drugs. For example, liver metabolism, kidney function, nerve transmission, or the functioning of bone marrow all decrease with age. In addition, in old age, there is a sensory decrease that increases the chances of errors being made in the administration of drugs.<ref>{{cite journal |vauthors=Merle L, Laroche ML, Dantoine T, Charmes JP | year = 2005 | title = Predicting and Preventing Adverse Drug Reactions in the Very Old | journal = Drugs & Aging | volume = 22 | issue = 5| pages = 375–392 | doi=10.2165/00002512-200522050-00003| pmid = 15903351 | s2cid = 26672993 }}</ref> The elderly are also more vulnerable to [[polypharmacy]], and the more drugs a patient takes, the higher is the chance of an interaction.<ref name="Rocío">García Morillo, J.S. ''Optimización del tratamiento de enfermos pluripatológicos en atención primaria'' UCAMI HHUU Virgen del Rocio. Sevilla. Spain. Available for members of SEMI at: [http://www.fesemi.org/grupos/edad_avanzada/reuniones/ponencias_ii_pppea/view ponencias de la II Reunión de Paciente Pluripatológico y Edad Avanzada] {{webarchive|url=https://archive.today/20130414224619/http://www.fesemi.org/grupos/edad_avanzada/reuniones/ponencias_ii_pppea/view |date=2013-04-14 }}</ref>

[[Genotype|Genetic factors]] may also affect the enzymes and receptors, thus altering the possibilities of interactions. {{Citation needed|date=November 2023}}

Patients with [[Hepatic disease|hepatic]] or [[renal disease|renal]] diseases already may have difficulties metabolizing and excreting drugs, which may exacerbate the effect of interactions.<ref name="Rocío" />

Some drugs present an intrinsic increased risk for a harmful interaction, including drugs with a narrow [[therapeutic index]],  where the difference between the [[Effective dose (pharmacology)|effective dose]] and the [[Lowest published toxic dose|toxic dose]] is small.<ref group="n.">The term effective dose is generally understood to mean the minimum amount of a drug that is needed to produce the required effect. The toxic dose is the minimum amount of a drug that will produce a damaging effect.</ref> The drug [[digoxin]] is an example of this type of drug.<ref name=":0">Castells Molina, S.; Castells, S. y Hernández Pérez, M. ''Farmacología en enfermería'' Published by Elsevier Spain, 2007 {{ISBN|84-8174-993-1}}, 9788481749939 Available from [https://books.google.com/books?id=FFBjWM-PKzkC]</ref> 

Risks are also increased when the drug presents a steep [[Dose-response relationship|dose-response curve]], and small changes in the dosage produce large changes in the drug's concentration in the blood plasma.<ref name=":0" />