Medicines and nutrients: understanding the dynamics

Medicines and nutrition have a two-way relationship: diet influences medicines and medicines influence nutritional status. These effects may contribute to symptoms that are treated with additional medicines instead of identifying and treating an underlying nutritional cause.

Poor nutrition increases the risk of disease including type 2 diabetes, cardiovascular disease, osteoporosis, frailty and impaired immune function. These are commonly managed with medicines.

Addressing the interactions between nutritional elements and medicines can contribute to safer prescribing, reduce adverse effects, and identify opportunities for non-pharmacological intervention or deprescribing.

Nutrition and medicines

Food influences medicines in several ways, including changes in stomach acidity, gastric emptying and drug metabolism. This can have an impact on efficacy and medicines toxicity.

A well-known example is grapefruit, which contains naturally occurring compounds that inhibit cytochrome P450 (CYP3A4) – an enzyme responsible for metabolising many medicines. Reduced metabolism results in higher circulating medicine concentrations, increasing the risk of toxicity.

Medicines affected include some statins, calcium-channel blockers and immunosuppressants. The interaction may persist for over 24 hours after grapefruit consumption.

Medicine administration advice also matters. Levothyroxine absorption is reduced by food, including dietary fibre, milk, soya products and coffee, as well as calcium and iron supplements. Patients should take levothyroxine on an empty stomach ideally 30 to 60 minutes before the first meal of the day, and separate calcium or iron preparations by at least four hours.

Calcium, magnesium and iron in food and supplements can bind with tetracycline and fluoroquinolone antibiotics in the gastrointestinal tract. A process called chelation forms insoluble complexes that reduce antibiotic efficacy. Reactions vary between different drugs; administration times should be spaced where appropriate.

Deficiencies and disease

Micronutrient deficiencies often produce non-specific symptoms that overlap with common conditions.

For example, vitamin B12 deficiency may present with fatigue, numbness and tingling, poor balance, memory problems or low mood. If the underlying deficiency is not recognised, treatment with medicines for neuropathic pain or antidepressants may be initiated while neurological damage progresses.

Iron deficiency symptoms include fatigue, breathlessness and cognitive dysfunction, which may be attributed to stress, ageing or poor sleep before iron deficiency is investigated. Similarly, non-specific symptoms such as fatigue, sleep disturbances, musculoskeletal pain, headaches and migraines may not be correctly identified as originating from a chronic magnesium deficiency.

Identifying the nutritional cause enables investigation of possible underlying pathology and targeted treatment.

Food allergy and intolerance

Some patients self-manage symptoms with medicines when dietary modification may be more appropriate. Recurrent abdominal pain, bloating or diarrhoea may prompt regular purchases of over-the-counter medicines when lactose intolerance, fructose malabsorption, coeliac disease or non-coeliac gluten sensitivity could be contributing.

Likewise, persistent urticaria, respiratory, nasal or gastrointestinal symptoms may warrant investigation for food allergy rather than indefinite symptomatic treatment.

Reduced absorption

Proton pump inhibitor (PPI) use is linked to a risk of vitamin B12, vitamin C, calcium, iron and magnesium deficiencies. PPIs reduce gastric acid secretion, but stomach acid is required to release vitamin B12 from dietary proteins.

Hypomagnesaemia has been reported, particularly after extended treatment with PPIs. Symptoms include muscle twitches, tremors, vomiting, tiredness and loss of appetite. Risk increases in those taking other drugs that may cause hypomagnesaemia (e.g. diuretics).

Long-term metformin therapy can reduce vitamin B12 absorption. Periodic vitamin B12 monitoring in patients with risk factors or symptoms suggestive of deficiency is recommended. Older adults are particularly vulnerable because they may already have reduced absorption.

Increased nutrient losses

Some medicines such as diuretics increase urinary excretion of minerals contributing to electrolyte disturbances.

Corticosteroids reduce calcium absorption while increasing calcium excretion, accelerating bone loss and increasing fracture risk.

Changes in appetite and food intake

Medicines frequently affect appetite and gastrointestinal symptoms. Nausea caused by antibiotics, opioids or GLP-1 receptor agonists may reduce food intake sufficiently to contribute to weight loss or malnutrition.

Conversely, corticosteroids, insulin and some antipsychotic medicines may increase appetite and promote weight gain, increasing cardiovascular and metabolic risk.

Older adults are particularly susceptible because reduced appetite, multimorbidity and polypharmacy often coexist.

Integrating nutrition into routine pharmacy practice

It is important to recognise when nutritional issues may be contributing to a patient's symptoms or medicine-related problems. Over-the-counter consultations and medicines reviews provide opportunities for earlier recognition.

Simple questions can help to identify potential problems before complications develop. Examples include:

• Are you taking any supplements?
• Have you noticed appetite changes?
• Have you changed your diet?
• Do particular foods trigger your symptoms?

Referral should be considered for further investigation where nutritional deficiencies, suspected food intolerance/allergy or medicine-related nutritional problems are identified.

References

1. Bailey DG, Dresser G, Arnold JM. Grapefruit-medication interactions: forbidden fruit or avoidable consequences? CMAJ. 2013 Mar 5;185(4):309-16.
2. Joint Formulary Committee. British National Formulary (BNF). London: BMJ Group and Pharmaceutical Press; 2026. Available at https://bnf.nice.org.uk/ (Accessed July 2026)
3. NHS.uk. Levothyroxine. 6 July 2026. Available at https://www.nhs.uk/medicines/levothyroxine/ (Accessed July 2026)
4. Health Improvement Scotland: Right Decision Service. Reducing Antibiotic Treatment Failure: a focus on tetracycline and fluoroquinolone antibiotics interaction. 26 April 2023. Available at https://www.rightdecisions.scot.nhs.uk/media/2bee2jgi/tetracycline-and-fluoroquinolone-nteractions-reducing-antibiotic-treatment-failure-20230426.pdf (Accessed July 2026)
5. National Institute for Health and Care Excellence. Anaemia – B12 and folate deficiency. Clinical Knowledge Summary. London: NICE; 2024. Available at https://cks.nice.org.uk/topics/anaemia-b12-folate-deficiency/ (Accessed July 2026)
6. National Institute for Health and Care Excellence. Anaemia - iron deficiency. Clinical Knowledge Summary. London: NICE; 2025. Available at https://cks.nice.org.uk/topics/anaemia-iron-deficiency/ (Accessed July 2026)
7. Ismail AAA, Ismail Y, Ismail AA. Chronic magnesium deficiency and human disease; time for reappraisal? QJM. 2018 Nov 1;111(11):759-763.
8. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev. 2015 Jan;95(1):1-46.
9. NHS.uk. Lactose Intolerance. 8 July 2026. Available at https://www.nhs.uk/conditions/lactose-intolerance/ (Accessed July 2026)
10. Simões CD, Sousa AS, Fernandes S, Sarmento A. Fructose Malabsorption, Gut Microbiota and Clinical Consequences: A Narrative Review of the Current Evidence. Life (Basel). 2025 Nov 6;15(11):1720.
11. National Institute for Health and Care Excellence. Coeliac disease. Clinical Knowledge Summary. London: NICE; 2025. Available at https://cks.nice.org.uk/topics/coeliac-disease/ (Accessed July 2026)
12. Shiha MG, Manza F, Figueroa-Salcido OG, Ontiveros N, Caio G, Jansson-Knodell CL, Rubio-Tapia A, Aziz I, Sanders DS. Global prevalence of self-reported non-coeliac gluten and wheat sensitivity: a systematic review and meta-analysis. Gut. 2025 Oct 28:gutjnl-2025-336304.
13. British Dietetic Association. Food allergy and food intolerance. March 2024. Available at https://www.bda.uk.com/resource/food-allergy-food-intolerance.html (Accessed July 2026)
14. Heidelbaugh JJ. Proton pump inhibitors and risk of vitamin and mineral deficiency: evidence and clinical implications. Ther Adv Drug Saf. 2013 Jun;4(3):125-33.
15. Medicines and Healthcare products Regulatory Agency. Proton pump inhibitors in long-term use: reports of hypomagnesaemia. Drug Safety Update. 2014. Available at https://www.gov.uk/drug-safety-update/proton-pump-inhibitors-in-long-term-use-reports-of-hypomagnesaemia (Accessed July 2026)
16. Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. Drug Safety Update. 2022. Available at https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk (Accessed July 2026)
17. Huxel C, Raja A, Ollivierre-Lawrence MD. Loop Diuretics. [Updated 22 May 2023]. StatPearls [Internet]. Available at https://www.ncbi.nlm.nih.gov/sites/books/NBK546656/ (Accessed July 2026)
18. Hsu, C.H., Hsu, C.L., Langley, A. et al. Glucocorticoid-induced osteoporosis: from molecular mechanism to clinical practice. Drugs Ther Perspect. 2024. 40:315-329.