12 Chapter 11: Pharmacology and Nutrition

Tracy Everitt; Megan Davies; and Sayuri Omori

Chapter 11 Learning Objectives 

At the conclusion of this chapter, students will be able to:

Learning Objectives

  • Understand the importance of food-drug interactions.
  • Describe where drug-nutrient interactions take place.
  • Describe the common foods and herbal products that can affect drug metabolism.
  • Report the consequences of certain medications on nutrition status.


Medical drugs, or medicines, are used to treat diseases, and to guarantee their safety and efficacy, they must be used in the appropriate way. Medicines usually contain active ingredients that react with the human metabolism in various ways, and certain dietary components frequently affect these properties of medicines. These so-called food-drug interactions occur when, for example, a dietary substance or food enhances, decreases or changes the activity of a drug. Typically, such interactions can be observed between drugs and foods (food-drug interactions) but also between drugs and herbs (herb-drug interactions) (White, 2010).


Food-drug interactions can be observed as a consequence of accidental misuse or insufficient knowledge on the substances involved in the interaction. In many cases, herbs, fruits, and alcohol can alter the efficacy of a given drug intervention with adverse consequences. In most cases, relevant food-drug interactions change the bioavailability of a medicine (White, 2010).


Less frequently discussed, but equally important, is that interactions can also go the other way around, i.e., medical drugs can also affect nutrition. As medicines and nutrients often employ the same mechanisms for absorption and are metabolically converted and excreted in the same ways, regular use of certain medications has the potential to negatively influence nutritional status The focus of this chapter is on food-drug interaction in older adults. We will discuss how foods can affect drug metabolism and vice versa how drugs can affect dietary intake and nutrition status (Lombardi et al., 2010). 

11.1 Food and Herbal Product Interaction with Drugs

Drug-drug interactions are well known, and particular attention is given to detailing these interactions, for example, on drug instructions or clinical databases and guides. Traditionally, there has been less discussion about food-drug interactions, but this has been growing. This is an important area as oral drug dosing is common. Food-drug interactions may arise due to changes in drug kinetics, intestinal transport, metabolism and distribution. The effect of standard meals on drug intake is tested during clinical trials, and recommendations are given based on these studies. Research has tried to quantify this effect, but the food intake can vary once in clinical settings. Several reviews have discussed this issue, focusing on older adults (Raats, de Groot & van Asselt, 2017).


The food-drug interaction is a complex subject, and the interaction can be within the digestive system (e.g., uptake hindrance or binding to drug) or metabolic inhibition (e.g., liver enzymes) (Nakanishi & Tamai, 2015). Several factors play a part in these interactions. The patient’s condition is important, such as age, pre-existing conditions, organ malfunction and polypharmacy. Emerging evidence is demonstrating that the gastrointestinal microbiome also factors in complicating the interaction between drug and food absorption and metabolism. In the book ‘Gut Microbiota,’ this interaction is studied further, addressing topics such as gut microbiota throughout the lifespan, gut microbiota in health and disease, and genetic and environmental influences on gut microbiota. Further information on how the gut microbiome affects food-drug interactions can be found in this book:  Ishiguro, E., & Campbell, K. (2023). Gut microbiota: interactive effects on nutrition and health. Elsevier. 


Supplements are often concentrated components of food items, such as garlic oil, or herbal products. Both food and herbal supplements, even when taken in highly concentrated forms, are often considered safe by patients. The regulation landscape around supplements is different than food and medical drugs and is complex and not uniform worldwide, which can confuse consumers (Thakkar et al., 2020). The use of supplements is common with 12–45% of people using both drugs and supplements and 6–29% are at risk of harmful interactions (Boullata, 2013). The risk of interactions may occur with common drug categories (antithrombotic, antidepressants, antidiabetics and sedatives).


A further difficulty is that patients can use herbal medications without health care providers knowing. Patients may omit reporting supplements or the use of herbal products, assuming they are safe (e.g., the common use of St John’s Wort, Ginkgo Biloba, Garlic, Ginseng, Echinacea, Saw Palmetto, Evening Primrose and Ginger). Bioavailability is not always negatively affected, as iron supplementation can increase the bioavailability of tetracycline. It is therefore important that there is an open dialogue and understanding between health care providers and patients.

Common Foods

Milk can reduce the bioavailability of drugs such as antibiotics (e.g., tetracycline, doxycycline and penicillin). A small amount of milk, such as in tea or coffee, has shown up to 49% reduction in tetracycline bioavailability (Jung et al., 1997). Tetracycline and doxycycline are not solely affected by milk, as food products can reduce the bioavailability significantly (Meyer et al., 1989). Health professionals often know these effects, but with these commonly used products, patients may fail to understand the extent of the effect the food can have on drug efficacy. For example, a morning dose before breakfast may be difficult for those patients with the habit of starting the day with a meal and supplements. If the patient takes their medication afterwards, it is likely to lead to an unwanted effect (such as slower recovery). Provision of instructions for patients that are clear and outline the potential consequences may improve adherence to prescriptive instructions and, ultimately, patient outcomes.



Other popular common food products include grapefruit and cranberry juice. The interaction between grapefruit juice and drugs has been known for decades, but in recent years, the number of reported cases has increased (Baily, Dresser & Arnold, 2013). For instance grapefruit juice can increase the amount of active medication in the body by blocking the enzymes that metabolize the medication (U.S. Food and Drug Association, 2021). In other cases, it may block the absorption transporters of the drug and decrease the amount of active drug in the body (U.S. Food and Drug Association, 2021). The degree of interaction that occurs between grapefruit juice and medications varies by person and by medication (U.S. Food and Drug Association, 2021).  Other fruit juices can also affect drug potency (the amount needed for a response), increasing and decreasing bioavailability (de Morais et al., 2013). Cranberry juice is frequently recommended for H. pylori and urinary tract infections; however, care must be taken. Although cranberry juice is considered safe and administered in high doses. Griffiths et al. (2008) reported a fatal case of hemopericardium (blood in the heart pericardium) and gastrointestinal hemorrhage caused by an interaction between cranberry juice and enzymes responsible for warfarin clearance (Griffiths, Beddall & Pegler, 2008). In this case, the patient was on a recommended 300–400 mL of cranberry juice per day, 6 weeks prior to the incident.


Garlic is traditionally used as a common food across many cultures but is also increasingly (self) prescribed as an alternative natural medicine supplement. Garlic has been studied extensively regarding its chemistry, pharmacology and clinical properties. The main research interests lie in the area of anticancer, antimicrobial and cardiovascular effects (Choo et al., 2020). The sulphur-containing compounds within garlic are considered responsible for the varied bioactive effects. Allicin is a precursor to several of these compounds; it is broken down enzymatically by allinase that is activated when garlic tissue is disrupted, cut or mashed. These unstable compounds are mostly found in fresh or freeze-dried garlic products. These compounds can cause an inhibition of CYP450 3A4 substrates—such as statin medications (e.g., atorvastatin, simvastatin) (Blalock et al., 2009).


Patients have traditionally taken garlic in hopes of a quicker recovery. In common folklore, blogs and healthcare websites, garlic has a reputation for helping with recovery from the common cold to cancer (Ansary et al., 2020); this is partially supported by evidence. Though garlic is considered food, a high dosage is commonly not recommended for older adults as there is evidence that garlic inhibits the effect of platelet activation, which in turn inhibits the mobilization of calcium stores. (Bradley, Organ & Lefer, 2016). Those that have slow blood clot formation or patients on anticoagulant, such as warfarin, should take care when taking garlic. In addition, several other herbal and natural products and foods interact with warfarin (Holbrook et al., 2005).

Herbal Products

Herbal products such as ginkgo, St John’s wort, ginseng and valerian are among the more commonly used herbal supplements and may be taken with and instead of conventional medication. The actions of ginseng are described as ‘adaptogenic’—an action that can be both CNS depressant and stimulant and help to balance bodily functions. This may seem contradictory; however, studies of isolated compounds support traditional uses (Lim et al., 2018). Preparations are normally well tolerated, but caution is advised. An example of a drug interaction is an adverse effect with antiplatelets or anticoagulants (Lau et al., 2009, Peterson, Bergien & Staerk, 2021).


 Another example is the effect of St John’s wort on antidepressants. Hyperforin, a component of St John’s Wort, has an effect on the liver and the small intestine. St John’s wort can cause a significant decrease in drug plasma concentration, such as phenprocoumon (anticoagulant). Other drugs affected by hyperforin intake include simvastatin, nifedipine, digoxin, verapamil, dabigatran, rivaroxaban, apixaban and edoxaban (Mouly et al., 2017).


What makes herbal products challenging is that the chemical composition varies between producers, and it is difficult to predict their potency. Analytical study on different St John’s wort products in the USA showed that the concentration of hyperforin ranged from 0.01 to 1.89%, where many contained lower amount than suggested for antidepressant effect (de los Reyes & Koda, 2002).

11.2 Effects of Drugs on Nutrition Status

Drugs Likely to Negatively Influence Nutritional Status

Medical drugs can negatively impact the bioavailability of nutrients and nutrition status in general (Van Zyl, 2011). Consequently, the regular use of certain medications has the potential to worsen nutritional status, and this effect of medication on overall nutrition status tends to be multifaceted (Van Zyl, 2011). Medication that has the greatest potential to impact nutrition status is usually taken over longer periods of time. According to Statistics Canada Health Reports, the most prescribed medications for older adults are:

  • beta-blocking agents
  • drugs for peptic ulcer and gastroesophageal reflux disease
  • hypnotics
  • sedatives
  • opioids
  • beta-lactam antibacterial
  • penicillin
  • antidepressants
  • lipid-modifying agents
  • antithrombotic agents
  • anti-inflammatory
  • antirheumatic
  • calcium channel blockers

 (Ramage-Morin, 2009).


A quarter of older adults were prescribed 10 or more mediations in 2021 (Canadian Institute for Health Information 2022). Side effects are unwanted physiological responses to medications and include impacts on nutritional status (Gervasio, 2010). Older adults both in the community and in a clinical environment are at higher risk due to frequently observed polypharmacy (Knight-Klimas & Boullata, 2004). Polypharmacy is when a person takes multiple medications for one or more conditions. Frequently, medication negatively affects dietary intake by direct action on digestion and metabolism, affecting an older adult’s nutritional status by decreasing dietary intake and bioavailability of essential nutrients, e.g., vitamins, trace elements and electrolytes (Van Zyl, 2011). With the aim to maximize drugs’ absorption, many drugs have detailed instructions regarding dietary intake (e.g., several antibiotics, antifungals), altering the usual routine of eating and meal frequency which can decrease overall energy and nutrient intake, often observed in older populations (Brixner et al., 2006). Anorexia is a frequently observed side effect of drug therapy leading to a reduction of body weight, and older adults with pre-existing poor nutrition status must be closely followed when starting drug therapy with medicines related to hypogeusia and dysgeusia (e.g., metformin, levodopa, lithium) (Brixner et al., 2006).

Drug Interaction in Older Adults

By direct action of the digestive tract, medications frequently disturb the nutrition status of older adults (Brixner et al., 2006). Some frequently used medicines lead to certain symptoms. The most common are listed in Table 11.2.1.


Table 11.2.1. Common symptoms of frequently used medications among older adults.

Frequently used Medications 
Anticholinergic drugs  


Antipsychotic drugs






Nausea and Vomiting 



Selective serotonin reuptake inhibitors

Taste Abnormalities 







Tricyclic antidepressants


Olfactory Disturbances 
ACE inhibitors 



Calcium channel blockers






Decreased gastrointestinal motility 
Tricyclic antidepressants 


Broad-spectrum antibiotics 

Proton-pump inhibitors



Magnesium salts




Even the unpleasant smell of medication can reduce food intake. Further, drugs have the potential to negatively affect the absorption of vitamins or trace elements due to alterations in gastric acidity, diminishing nutrition assimilation, negatively impacting gut microbiota and leading to inflammation of the mucosa of the digestive tract (e.g. antacids, antibiotics, laxatives, anti-inflammatories, hypoglycemic drugs, lipid-lowering drugs, antidepressants, diuretics). When the negative action of drug therapy on nutrition status is likely to become clinically significant, the nutritional deficiency can be corrected with the use of dietary supplements (e.g. pyridoxine administered with isoniazid, an antibiotic used for the treatment of tuberculosis). Even though classic symptoms of nutrient deficiency syndromes are not often seen, nutrient insufficiencies can be still related to clinical manifestations. In several cases, the negative impact of a certain medication on nutrition status is acknowledged as adverse drug effects. For example, drug-induced osteomalacia is a result of some antiepileptic drugs negatively affecting vitamin D metabolism (Pascussi et al., 2005). Further, drug-induced hepatotoxicity and hyperammonemia (e.g., valproic acid, a drug to treat certain types of seizures) are thought to be a consequence of a lack of carnitine (Van Wouwe, 1995). It is still unclear what drug-nutrient interaction dietary supplementation is a feasible solution, and this is a field which needs more research in order to be able to recommend prophylactic nutrient supplementation. It should also be mentioned here that there are some drugs that are related to improvements in nutrient status (Drain et al., 2007). Older adults are likely to use medication that can impair hearing, vision, memory and mobility, resulting in a lack of compliance or wrong dosing (Van Zyl, 2011). This indirect drug action can then affect food intake by impairing the ability to buy and prepare food or to eat without assistance (Tawara et al., 1997).


Researchers have determined that 90% of all oral medications contain at least one ingredient capable of causing an adverse reaction (Reker et al., 2020). Inactive ingredients are broadly defined as any component of a drug product other than an active ingredient. These components are not intended or expected to have a direct biological or therapeutic effect but instead are added to alter the physical properties of an oral solid dosage form to facilitate absorption or to improve stability, taste, and appearance (Reker et al., 2020). Increasing numbers of clinical reports have seen adverse reactions triggered by an inactive ingredient in a medication (Reker et al., 2020). These are the most common symptoms in the form of an allergy or an intolerance to inactive ingredients. Intolerance to an inactive ingredient can cause symptoms through mechanisms such as malabsorption, which causes gastrointestinal symptoms (Reker et al., 2020).

Drugs Likely to Increase Body Weight

For a sense of completeness, it should be briefly mentioned here that drugs can also increase body weight. For example, it is well known that active drug ingredients can change the function of an older adult’s metabolism. Undesirable metabolic adverse effects are frequently observed for antipsychotic drugs which can lead to weight gain, hyperglycemia and dyslipidemia (Yoon et al., 2010), alterations that have been related to the use of second-generation antipsychotics (Williams, Alinejad & Williams, 2010). Other frequently used medications such as beta-blockers and steroids have been also shown to increase body weight (Alam, Voronovich & Carley, 2013). The observed increase in body weight is not necessarily always due to an increase in body fat. There are several potential mechanisms which can explain medication-related weight gain: increased appetite (steroids and antipsychotics), fluid retention (pioglitazone), increased fat storage (insulin, stimulating the growth of fat cells), slowed metabolism (beta-blockers) and difficulty exercising (antihistamines can make you sleepy, amitriptyline associated with breathing difficulties).Although maintenance or even an increase in body weight in older adults can be beneficial, these drug-induced changes in body composition and metabolism are usually not positive as they are not associated with positive changes in muscle mass or physical function (2004). If not carefully considered, this may lead to sarcopenic obesity and the under-identification and under-treatment of those who are overweight or obese and are also malnourished (Bell et al., 2021). 


Clinical specialists must recognize that medical drugs and dietary intake interact with each other and can affect the health of older adults. During the last years, these interactions have received more attention and have been better understood in clinical practice. An acknowledgement and broad understanding of the problem are the most important steps towards optimized older adult care. To resolve negative drug actions on nutrition status and vice versa, clinicians must employ a systematic approach and take the time to listen to the perspectives of the older adult. In circumstances where sufficient information on the interactions is available, adverse interactions can be anticipated and potentially mitigated. Healthcare staff should be trained to detect food-drug interactions in order to give older adults instructions on medication, food and beverages. Appropriate specialists must estimate a patient’s clinical presentation in order to recognize whether the nutritional status is deteriorating or associated with a drug-induced complication. A change in dosage or a change in medications within the same therapeutic category or a change in dietary habits must be considered. Finally, and most importantly, health care providers should work together with older adults to deliver food and medicine regimens that are most likely to improve older adult outcomes, experiences and quality of life. 


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