How medication affects nutrient intake

Food-drug interactions are a very complex subject. On the one hand, many different diseases require medication, while on the other hand the use of certain meds can affect the patient’s food intake and, in turn, cause nutritional problems.

If we also keep in mind that the number of available drugs or medications is increasing exponentially and also becoming exceptionally sophisticated (e.g. drugs like the new ‘Biologicals’ or ‘Biologics’ which are manufactured by biological processes rather than chemical ones), then it is evident that this is an important subject which we should all be aware of.

Effects of food-drug interactions

Broadly speaking, food-drug interactions can cause the following effects:

  • Change the way in which a patient reacts to a given medication
  • Increase the risk of a toxic drug reaction
  • Change the nutritional status of the patient (for example by reducing nutrient absorption)

(Mahan LK et al, 2011)

Meds that reduce nutrient absorption

Certain meds can affect how the human body absorbs nutrients, which will then affect the nutritional status of the patient. Typical examples of such reactions are:

1) Chelation Reactions:

  • Chelation reactions that occur with minerals and trace elements. During chelation processes, a chemical such as a pharmacological drug binds to a mineral making it more difficult to be absorbed by the human digestive system.
  • Tetracyclines and ciprofloxacin (different types of antibiotics) can chelate or bind calcium in dairy products or food supplements and prevent the body from absorbing the calcium. The availability of other minerals such as iron, magnesium and zinc can also be reduced by chelation (Mahan LK et al, 2011).


To avoid chelation of calcium, magnesium, iron and zinc when you take tetracycline and ciprofloxacin antibiotics, take mineral supplements at least 2 tot 6 hours after the antibiotics.

2) Adsorption

  • Adsorption (not to be confused with absorption), involves a process where chemicals such as pharmacological drugs bind to nutrients so that they cannot be taken up in the body.
  • The so-called bile acid sequestrant cholestyramine, which is used to treat raised blood cholesterol levels, also adsorbs fat-soluble vitamins A, D, E and K, so that long-term use of cholestyramine can lead to deficiencies of these vitamins (Mahan LK et al, 2011).


Patients taking cholestyramine to lower their blood cholesterol levels are advised to take vitamin supplements, especially if the patient has to take the drug more than once a day.

3) Decreased absorption

  • Mineral oil (which is used as a laxative) also reduces the adsorption of fat-soluble vitamins A, D, E and K, which can cause deficiencies especially if the mineral oil is used for long periods (Mahan LK et al, 2011).
  • Example of Mineral Oil: Liquid Paraffin (over-the-counter medication)


Take the mineral oil in the morning and vitamin supplements at least 2 hours later. If you are self-medicating yourself with Liquid Paraffin because of constipation, keep in mind that you may develop vitamin A, D, E and K deficiencies is you use this type of laxative all the time.

4) Speeds up transit time:

  • Many laxatives (prescription and over-the-counter) speed up the so-called ‘transit time’ in the gut and can cause diarrhoea which can lead to massive losses of calcium and potassium and in turn cause electrolyte imbalances and even death.


Do not take laxatives indiscriminately, particularly for purposes such as weight loss. The function of a good laxative is to promote gentle bowel movements and comfortable evacuation. If the laxative you are using causes cramps, pain and diarrhoea, then it is advisable to discontinue it immediately as this can cause the above mentioned electrolyte imbalances or be fatal. Constant misuse of laxatives can also lead to loss of normal bowel function. Keep in mind that the absorption of contraceptive pills can also be affected by the use of harsh laxatives which cause constant diarrhoea.

5) Drugs that cause diarrhoea

  • A variety of drugs may cause diarrhoea which can also lead to all the problems listed above under Point d).
  • Any drug that contains sorbitol (e.g. syrups or solutions of furosemide which is used as a diuretic), or valproic acid (used to treat epilepsy, mania in bipolar disorders and migraine), or carbamazepine (used to treat epilepsy, mania in bipolar disorders and trigeminal neuralgia) can cause diarrhoea, which can interfere with the absorption of nutrients, other medications including contraceptives, and cause dehydration due to excessive water losses (Mahan LK et al, 2011).


If you develop diarrhoea when you start taking any of the above mentioned drugs, contact your prescribing doctor so that he/she can treat your diarrhoea and either adjust your dosage  or prescribe other treatment.

6) Drugs that reduce nutrient absorption by changing the pH and/or gastric acid secretion in the gut

  • All the medications used to treat excessive acid production, gastric and duodenal ulcers, and GORD (gastro-oesophageal reflux disease), are designed to reduce the amount of acid that is secreted in the stomach and change the pH from highly acidic to more alkaline (Mahan LK et al, 2011).
  • Both H2-receptor antagonists such as ranitidine and proton-pump inhibitors like omeprazole can impair the absorption of vitamin B12 and cause vitamin B12 deficiencies which can in turn lead to megaloblastic anaemia.


If you need to take medications to reduce stomach acid for long periods of time, it may be a good idea to ask the prescribing doctor to test you for vitamin B12 deficiency at regular intervals.

7) Meds that damage the lining of the digestive tract and hamper absorption

  • A number of medications, particularly chemotherapy drugs, non-steroidal anti-inflammatory drugs (NSAIDs) and protracted antibiotic therapy have the potential to damage the lining of the gut resulting in malabsorption of nutrients.
  •  NSAIDs can cause colitis leading to bloody diarrhoea, weight loss and iron deficiency anaemia (Mahan LK et al, 2011).
  •  Example of NSAIDs: Ibuprofen which is used in many drugs for the treatment of pain, arthritis, etc.


If you are on anticancer therapy or taking NSAIDs or require long-term antibiotic treatment and you develop diarrhoea and start losing weight, please report these symptoms to your doctor so that he/she can check if you are suffering from deficiencies caused by malabsorption of nutrients (e.g. iron deficiency anaemia).

8) Drugs that interfere with fat absorption and prevent the uptake of fat-soluble vitamins

  • Slimming drugs containing orlistat such as Xenical and Alli, interfere with lipase enzyme activity and reduce fat  uptake by the gut. This can also reduce the uptake of fat-soluble vitamins A, D, E and K, with resulting deficiencies (Mahan LK et al, 2011)..


Patients using orlistat for slimming purposes should use a vitamin supplement containing vitamins A, D, E and K and take such supplements at least 2 hours before or after the orlistat. Regular monitoring of vitamin status may also be advisable.


In view of the fact that so many drugs can affect how your body absorbs essential nutrients such as vitamins and minerals, the following precautions should be kept in mind:

  • Always read the Patient Information Leaflet or the Packages Insert that are included with your pills or capsules. If you do not understand something in these brochures, ask your doctor or pharmacist to explain things to you.
  • Be on the lookout for side-effects and if they should occur (especially diarrhoea) get help from your doctor to prevent the development of deficiencies and imbalances.
  • Don’t stop taking a prescribed medication unless your doctor has told you to do so because some of these drugs have been prescribed to prevent/treat serious conditions like cancer or epilepsy. The physician will decide what treatment you require to prevent nutrient malabsorption or to treat deficiencies.

Dr IV van Heerden, DietDoc


(Mahan LK et al (2011). Krause’s Food & the Nutrition Care Process. Ed. 13. Elsevier, USA)

Read more:

Medication can cause weight gain
Psychiatry: drug-food interactions

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