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Oral administration of drugs: Drug - food interactions

Nélida Barrueco,1 Cecilia Martínez Fernández-Llamazares,2 Esther Durán,2 María Teresa Martínez Marín,3 Cristina Relaño García4
4th Year Hospital Pharmacy resident, Pharmacy Department; 2Hospital Pharmacist, Pharmacy Department; 3Nurse, Pediatric cardiology unit; 4Chief Head of Nursing Department. Gregorio Marañón General University Hospital. Madrid, Spain

Mail delivery: Nélida Barrueco Fernández. Servicio de Farmacia. H.G.U. Gregorio Marañón. C/ Dr. Esquerdo 46, 28007 Madrid, España

Manuscript received by 25.5.07
Manuscript accepted by 9.8.07

Index de Enfermería [Index Enferm] 2008; 17(1): 53-57








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Barrueco, Nélida; Martínez Fernández-Llamazares, Cecilia; Durán, Esther; Martínez Marín, Mª Teresa; Relaño García, Cristina. Oral administration of drugs: Drug - food interactions. Index de Enfermería [Index Enferm] (digital edition) 2008; 17(1). In </index-enfermeria/v17n1/6555e.php> Consulted by





Background: The easiest, safest and cheapest way to administrate drugs is by mouth (PO). Nevertheless, there may be interactions, either with other drugs or with food, which can modify efficacy and security of the drug itself.
Objective: the development of a nursing information program about the administration of drugs PO.
Method: we selected the most used drugs corresponding to the pediatric cardiology area, looking for the best administration possible with or without food or medicinal products, and any other aspect related to their administration PO.
Results: we elaborate the following table with all the information collected.
Discussion: hospital pharmacists have recently been integrated in the multidisciplinary team that take care of patients, and so now they have the opportunity of developing different pharmaceutical care programs, sanitary education and information focused on the prevention of medication errors, the secure use and the diminish of any risks associated to any pharmacological treatment.
Medical prescriptions usually do not specify timetable of way of administration of drugs, letting nurses the responsibility of their organization. That is why they must be well informed of how and when drugs must be administrated, so we can improve the security and decrease the risks associated to them.
Key-words: Oral administration, Pharmaceutical Care, interactions, food-drug interactions, drug administration schedule.









    The oral route of administration is the easiest, safest and economical means of administering drugs, and is the route of choice whenever possible. However, in order to obtain the highest safety and efficacy from the use of drugs it is necessary to consider aspects such as bioavailability, GI tolerance, interactions with drugs used concomitantly or not, and drug-food interactions,1 as well as identification of the most easily-recognized adverse effects. Another major aspect to be taken into account, although not discussed in this paper, is the adjustment of oral administration to the dose interval indicated by the medical prescription, since dose intervals need to be strictly respected in some cases, regardless of their relation with food.
    A drug-food interaction may be described as the one resulting in the occurrence of a pharmacological, toxic or therapeutic effect of lesser or greater intensity than expected, which takes place as a consequence of the action or presence of food on drugs or vice versa.
2 Drug-food interactions are not so frequently detected as drug-drug interactions. Nevertheless, their potential frequency may be higher, since foods are the substances most frequently associated with drug administration. The highest risk situations are those involving drugs with a narrow therapeutic margin and drugs for chronic use.
    Interactions between drugs not administered simultaneously are not included in this work, since that would involve, within the pharmaceutical care tasks, several different levels of healthcare, including physicians and nurses, in the search for the greatest efficacy and safety in the use of drugs. It seems more appropriate to address this aspect on a per-patient basis in the medical prescription systems, perhaps by including them as prescription aides within the framework of electronic prescription systems.
    Two types of interaction production mechanisms can be identified: pharmacokinetic and pharmacodynamic.
    Pharmacokinetic interactions: the drug's ordinary pharmacokinetics may be modified and drug concentrations may be altered in the body, affecting therapeutic activity and the occurrence of adverse reactions, which may decrease or increase. Action may take place at different levels: in the release (pH modification, altered motility and modified GI secretions) and in the absorption (delayed gastric emptying, increased motility, altered effect of the first hepatic passage) that determine modifications to the speed of absorption and/or total amount absorbed. There may also be interactions in the delivery (displacement from plasma protein binding), metabolism (inhibition or activation of the hepatic enzymes involved in metabolism) and excretion (modified urinary pH).
    Pharmacodynamic interactions: the interaction is produced at the level where the drug acts; although these are fewer when compared to pharmacokinetic interactions, they may be more serious.
    Not only are there interactions with food, but also -and increasingly- with medicinal products, medicinal herbs and dietary supplements, such as St. John's wort, grapefruit juice and certain vitamins and minerals.
    Medical prescriptions usually do not specify whether a drug should be administered with or without food, or if it should be administered separately from other drugs or medicinal products, giving nurses the responsibility for arranging drug administration. Thus, nurses frequently give drugs and food simultaneously. Although this improves swallowing, gastric tolerance and compliance, it implies some inconveniencies such as irregular administration intervals (failing to strictly adjust to dosing intervals) and the possibility of drug-food interactions.4
    Increasing the training period in hospital pharmacy for intern resident pharmacists to four years, together with a clinical orientation in the last year, has enabled the hospital pharmacist to better integrate into the multidisciplinary team that assists patients, thus favoring pharmaceutical care and healthcare education tasks. The number of questions on drug administration observed in the pediatric cardiology unit during fourth year resident pharmacists' rotation encouraged us to design this program, shown below, the product of their integration into this multidisciplinary team.
    Objective: Developing a nurse-oriented informative program on a pediatric cardiology unit on the administration of drugs PO and their relation to food.


    The most used drugs (in terms of the number of units consumed) in the pediatric cardiology unit have been selected. This drug selection accounts for 82% of the drugs used PO in the unit during a one-year study period. Each drug is reviewed for available pharmaceutical formulations; the possibility of preparing compounded medications or dose adaptations, both in liquid and solid form; their administration in relation with food and medicinal products; and their most frequent adverse reactions. The bibliographical sources used are technical data sheets5 and the drug database Drugdex® from Thomson Micromedex® Healthcare series6 and Lexi-Comp®.7


    A document with a total of 28 drugs is drafted, each specifying:
-Available commercial formulations and possibility of making compounded formulas or dose adaptations.
-Most frequent adverse effects.
-Administration in relation with food.
-Other information of interest in drug administration, such as the possibility of handling oral formulations, the most suitable time for administration and the need to separate the administration from that of other drugs.
    A meeting was held with nurses and with the unit supervisor when we released the document. Proposed modifications and suggestions were included, and the definitive information is shown in table 1. At the same time we established with the nurses a one-month period for them to study the information included in the program so as to, following said period, assess the amount of questions received and check if a reduction in their number have occurred.


    Hospital pharmacists have recently joined multidisciplinary teams. From that position, they have the opportunity to develop different pharmaceutical care, health education and information programs oriented to preventing drug-related problems, thus promoting safer use and decreasing risks associated with any pharmacological therapy.
    Nurse-oriented programs are included in the healthcare education/information programs for pharmacists. The objective of these programs is to raise nurses' level of information and knowledge of drugs, increasing their participation in order to optimize drug use. In the case shown here, and due to the integration of the resident pharmacist into the pediatric cardiology unit, the program is specially oriented to the nurses on that unit, and so includes the drugs most used by nurses.
    The nature and consequences of drug-food interactions will depend on the characteristics of the drug, the food and the patient.
    In the case of drugs with a narrow therapeutic index, with a steep dose-response curve or those requiring dose titration, small variations may have serious consequences.
    In the case of food, both the volume ingested and the composition (proportion of fats, proteins, fiber, electrolytes, etc.) are important. For example, fiber-rich foods reduce the amount of digoxin absorbed, or a variation in potassium dietary consumption may modify the efficacy and safety of spironolactone.
    Lastly, some of the influencing factors in the case of patients are comorbidities, concomitant medication, age and gender. Examples include the alterations in furosemide and digoxin absorption when they are administered with sucralfate.
    Of the 28 drugs reviewed, only seven require administration under fasting conditions. "Fasting" means one hour prior to or two hours following the intake of any food. Any administration not complying with this definition cannot be considered to fit fasting conditions.
1 Eleven drugs may be administered with or without food, and eight of them should be given with food. That is to say, most of the drugs reviewed may be administered with food; additionally, in the case of ten of them, administration with food reduces gastric discomfort. Thus, as a rule of thumb, drug administration may coincide with food intake in order to minimize stomach upset, facilitate swallowing and favor treatment compliance, but it is essential to be aware of the exceptions.
    It should be noted that drug administration with food or under fasting conditions should not imply the disruption of the dose intervals established for each drug. In other words, administration timetables should not be modified simply to facilitate drug administration with food; on the contrary, based on dose intervals, one should attempt to give food together with the drug.
    Regarding the co-administration of other drugs or medicinal products via the oral route, most drugs should be administered separately (at least two hours apart if not otherwise specified) from resins, aluminum salts, magnesium and calcium.
    Medical prescriptions usually fail to indicate time and route of administration, making nurses responsible for those arrangements. Thus, nurses should be informed as to how and when drugs are to be administered. They should know the best time for administration, the need to give a drug with or without food or whether to separate the administration from another concomitant drug. It is also important that they know how to handle pharmaceutical formulations in order to make swallowing easier and how to have liquid or solid formulas adapted to pediatric dosing. These compounded formulas, prepared by the pharmacy departments, make administration easier and dosing more precise. Lastly, knowledge of the most common adverse reactions promotes rapid detection and problem resolution, since nurses are the healthcare personnel who spend the most time with patients.


    Drawing up various information programs oriented to the healthcare staff responsible for administering drugs seems to be a need in this field, given the hot questions received regarding drug administration in these patients. These programs are needed because they provide nurses with useful information for their daily tasks and may contribute to reducing the occurrence of interactions and other drug-related problems.


1. Delgado O, Puigventós F, Serra J. Administración de medicamentos por vía oral. Med Clin 1997; 108: 426-35.
2. Madurga M. Interacciones de los alimentos con los medicamentos. Auladefarmacia [serie en Internet]. 2004 Jun; 6(1): 40-46. Disponible en: <https://www.auladelafarmacia.org/> [citado 8 Feb 2007].
3. Ohnishi N, Yokoyama T. Interactions between medicines and functional foods or dietary supplements. Keio J Med. 2004; 53(3): 137-50.
4. Rabadán MT, Flores MJ, Cayuela J, Cevidades MM, Valvuena R, Ruiz MT, Rodríguez M, Vervantes MD. Interacciones medicamentosas en la administración de fármacos dentro del proceso de enfermería. Enfermería Global [Serie en Internet]. 2002 1: 1-23. Disponible en: <https://www.um.es/eglobal/> [citado 21 Feb 2007].
5. Fichas técnicas de medicamentos. Disponible en: <https://sinaem.agemed.es:83/presentacion/principal.asp> [Fecha última consulta 17 diciembre 2006].
6. MICROMEDEX Healthcare series, Thomson MICROMEDEX, Greenwood Village, Colorado. Disponible en:
<http:/www.micromedex.com/products/drugdex/> [Fecha de última consulta: 24 enero 2007].
7. Lexi-Comp Online TM, Hudson, Ohio: Lexi-Comp, Inc.; 2006. Disponible en: <https://online.lexi.com/crlsql/servlet/crlonline> [Fecha de última consulta: 24 enero 2007].
8. Schmidt LE, Dalhoff K. Food-drug interactions. Drugs 2002; 62(10): 1481-502.


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