Absorption Phase Drug Interactions

Drug Interactions

Drug interactions occur when one drug’s pharmacokinetic, pharmacodynamic or toxicological characteristics are influenced by a second substance. Usually, the second substance is another drug—termed a “drug-drug interaction”—but it can also be a component of the diet (such as proteins, fats, minerals, vitamins). In describing these interactions we use the terms “victim” and “perpetrator”. The victim is the drug whose PK, PD or tox characteristics are changed whereas the perpetrator is the drug (or sometimes a nutrient) who causes the change. In many instances, both drugs are affected and the victim/perpetrator model breaks down.

Absorption Phase Drug Interactions Overview

Absorption phase drug interactions can occur through physicochemical mechanisms, competition for transporters, or alterations in GI physiology. The absorption phase is particularly susceptible to drug interactions because the drug is exposed to a complex and dynamic environment in the gastrointestinal (GI) tract where there is a high concentration of competing substances. The presence of other drugs and dietary components can significantly influence the extent and rate of drug absorption, ultimately affecting therapeutic outcomes for vulnerable drugs.

Physicochemical Interactions

Physicochemical interactions occur because drugs interact directly with other substances inside the GI tract before absorption can occur. In most cases, these interactions reduce drug absorption because they result in complexes that have poor solubility (see Factors Affecting Absorption for an overview of why solubility is important). Unlike other biological interactions, physicochemical interactions can often be overcome simply by separating the administration times of the interacting substances so they are not present inside the GI lumen at the same time.

Metal Ion Complexation

Metal ions such as calcium (Ca²⁺), magnesium (Mg²⁺), iron (Fe²⁺/Fe³⁺), aluminium (Al³⁺), and zinc (Zn²⁺) can form insoluble chelate complexes with certain drugs in the GI tract. Chelation occurs when the drug molecule acts as a ligand, donating electron pairs to coordinate with the metal ion, creating a stable ring structure. These chelate complexes are generally too large and too polar to cross the intestinal epithelium—drug absorption cannot occur. The strength of chelation depends on the specific metal ion.

It's fairly easy to see where someone might encounter these interacting minerals in their diet or supplements (eg calcium in dairy foods, magnesium supplements) however a less obvious source is in medications. Mineral antacids are a class of medications for acid reflux and are salts of calcium, magnesium or aluminium.

• Tums®—calcium carbonate
• Maalox®—magnesium hydroxide and aluminium hydroxide
• milk of magnesia—magnesium hydroxide
• Gaviscon®—aluminium hydroxide and magnesium carbonate
• Pepcid Complete®—calcium carbonate and magnesium hydroxide
• Mylanta®—aluminium hydroxide and magnesium hydroxide

The most clinically significant examples involve the tetracycline and fluoroquinolone classes of antibiotics. Tetracyclines (eg tetracycline, doxycycline, minocycline) contain multiple hydroxyl and carbonyl groups that readily chelate divalent and trivalent cations. When tetracyclines are taken with dairy products (rich in calcium), antacids (containing aluminium, magnesium, or calcium), or iron supplements, their absorption can be reduced by 50-90%. This dramatic reduction can lead to therapeutic failure in treating infections. Similarly, fluoroquinolones (eg ciprofloxacin, levofloxacin, moxifloxacin, norfloxacin) chelate metal ions through their carboxylic acid and ketone groups. Depending on the specific victim antibiotic and the perpetrator metal ion, AUC can be reduced by as much as 90%.

Bisphosphonates (eg risedronate, alendronate), used for osteoporosis treatment, are particularly susceptible to chelation. Alendronate and risedronate can form complexes with calcium, magnesium, iron, and aluminium, reducing their already poor oral bioavailability (typically <1%) to zero. This is why bisphosphonates must be taken on an empty stomach with plain water, and patients must wait at least 30 minutes before consuming food, beverages (other than water), or other medications.

Levothyroxine, synthetic T4 thyroid hormone used for treating hypothyroidism, can also chelate iron and calcium, reducing thyroid hormone replacement efficacy if not properly separated from supplements or calcium-rich foods.

Common drug-metal ion interactions and management strategies.

Drug	hours BEFORE minerals	hours AFTER minerals
ciprofloxacin	2	6
moxifloxacin	4	8
norfloxacin	2	2
levofloxacin	2	2
ofloxacin	2	2
nalidixic acid	2	2
doxycycline	4	2
tetracycline	4	2
minocycline	4	2
risedronate	0.5
alendronate	0.5
levothyroxine	4	4

Overcoming this interaction is fairly straightforward: avoid taking antacids or eating foods with lots of minierals close to administration of the drug. The table below summarises the recommended intervals for some of these drugs

Cholestyramine

Cholestyramine is a bile acid sequestrant (aka bile acid binding resin, BABR) used primarily to lower cholesterol levels. It is a large, polymeric, non-absorbable anion exchange resin that binds bile acids (cholesterol) in the intestinal lumen, preventing their reabsorption and promoting their faecal excretion. However, cholestyramine is not selective for bile acids and can easily bind many acidic and neutral drugs through electrostatic interactions. Once bound to the high molecular weight cholestyramine, the drug cannot be absorbed and passes through in the feces. Unlike metal ions which form specific chemical chelate species, cholestyramine acts something like a sponge onto which many drug molecules adsorb.

Cholestyramine is a very aggressive BABR and has very non-specific binding to other drugs. Some include...

• warfarin
• digoxin
• levothyroxine
• thiazide diuretics
• statins and fibrates
• penicillins
• fat-soluble vitamins (A, D, E and K)
• sodium valproate
• mycophenolate
• steroids (including corticosteroids and oral contraceptives)

You should take note that many of these are high risk and/or carry serious clinical consequences if less drug is absorbed. For example, the interaction with warfarin is concerning because it can lead to subtherapeutic anticoagulation and increased risk of thromboembolism.

To minimize these interactions, it's commonly recommended to take other medications at least 1 hour before or 4-6 hours after taking cholestyramine. For patients who are not able adhere to this recommendation and who must have a BABR, an alternative binder can be suggested. Colestipol is a less aggressive binder with fewer drug interactions although the tradeoff is somewhat lower efficacy. Colesevelam is weaker still but is the least likely to affect absorption of other drugs (although caution is still warranted).

Sucralfate

Sucralfate is a rarely-used drug for treating duodenal ulcers. Pharmacologically, it acts by forming a barrier on top of the ulcer to facilitate healing. The mechanism by which it interferes with absorption of other drugs is unclear but many sources suggest it is realted to this pharmacological action—the sucrulfate layer on top of the mucosa physically blocks other drugs from accessing the enterocyte for absorption. Sucralfate decreases absorption of warfarin, digoxin, levothyroxine, bisphosphonates, fluoroquinolones and tetracyclines

Fat

Dietary fat has several effects on drug absorption. High-fat meals stimulate bile secretion and slow gastric emptying. Fat can also affect absorption by acting as a vehicle of dissolution for lipophilic drugs.

For highly lipophilic drugs, the presence of dietary fat can dramatically increase absorption. This is the case for the antifungal agent griseofulvin, the immunosuppressant cyclosporine, some HIV protease inhibitors and non-nucleoside reverse transcriptase inhibitors. These drugs are often formulated with lipid excipients or patients are instructed to take them with meals to ensure consistent, adequate absorption.

As discussed in Anatomy and Physiology of Drug Absorption, some highly lipophilic drugs can be absorbed into the intestinal lymphatic system when taken with fat-containing meals. Cannabinoids (THC and CBD), some antiretrovirals (lopinavir, efavirenz), and griseofulvin can undergo lymphatic absorption, which allows them to bypass hepatic first-pass metabolism. For these drugs, taking them with fatty meals not only enhances overall absorption but also changes the route of absorption, potentially increasing the fraction of drug that reaches systemic circulation unchanged.

Transporter Interactions

Transporter-mediated drug interactions occur when one drug affects the function of membrane transporters responsible for drug uptake or efflux in the intestinal epithelium. These interactions can significantly alter drug bioavailability by either enhancing or reducing the amount of drug that successfully crosses the intestinal barrier. Unlike physicochemical interactions that can be managed by separating dosing times, transporter interactions require more careful consideration as the effects persist as long as the inhibitor or inducer remains in the system.

Uptake transporters move drugs from the intestinal lumen into enterocytes as discussed in Anatomy and Physiology of Drug Absorption. These are extremely helpful for absorption of drugs with poor plasma membrane diffusion however when multiple drugs require the same transporter, interactions can occur.

Efflux transporters pump drugs out of enterocytes back into the intestinal lumen, thereby reducing net drug absorption. The most clinically important efflux transporter is P-glycoprotein (P-gp, MDR1, ABCB1), located on the apical membrane of enterocytes. As discussed in Factors Affecting Drug Absorption, P-gp has broad substrate specificity and its activity can be modulated by inhibitors and inducers, leading to significant drug interactions.

Peptide Transporters

High-protein meals can affect drug absorption through a number of mechanisms including stimulating gastric acid secretion and delaying gastric emptying as discussed below. The small products of protein digestion in the GI tract can also have direct influence on the active transporters that take up some drugs. The peptide transporters PepT1 and PepT2 (solute carrier family 15A members 1 and 2, SLC15A1/2) are responsible for absorbing small peptides (di- and tripeptides) from dietary protein, but also transport certain drugs that have peptide-like structures. Examples include β-lactam antibiotics (penicillins and cephalosporins), ACE inhibitors, and the antiviral valaciclovir. When taken with protein-rich meals, dietary peptides can compete with these drugs for PepT1/2, potentially reducing their absorption. However, this interaction generally has only a minor effect on drug absorption because PepT1/2 are abundandant and have a very high capacity—not easily saturated. More significant interactions can occur with PepT1/2 inhibitors, although those that are used clinically—sulfonylureas and ARBs—are also fairly weak inhibitors.

Amino Acid Transporters

We've already seen how small products of protein digestion—di- and tripeptides—may affect drug absorption through the PepT1/2 transporters. Going one step further, the free amino acids produced by breakdown of the di- and tripeptide digestion, have transporters of their own. The large amino acid transporter 1 (LAT1) is involved in uptake of branched amino acids (valine, leucine, isoleucine) and aromatic amino acids (phenylalanine, tryptophan and tyrosine). It is also used by the anti-Parkinson's disease medication levodopa (which is a derivative of tyrosine) for uptake from the GI tract. When taken at the same time as protein, tyrosine and levodopa compete for LAT1 causing lower absorption of levodopa and worsen control of motor symptoms. This is usually most problematic for meals containing more than 30g of protein and patients are advised to take levodopa on an empty stomach or with a low-protein meal. Clinicians sometimes recommend a “protein redistribution diet” where patients have low-protein meals (<10g) during the day when symptom control is most crucial but high protein meal (>30g) in the evening.

Organic Anion Transporters

Organic anion transporting polypeptides (OATPs), particularly OATP1A2 and OATP2B1 expressed in the intestine, transport numerous drugs including statins, the antihistamine fexofenadine, and certain antibiotics. Flavonoids found in some fruit juices—grapefruit, orange, apple—inhibit intestinal OATPs. Co-injestion of an OATP substrate drug with fruit juice can decrease AUC of the drug by 85%.

P-glycoprotein

When a P-gp inhibitor is co-administered with a P-gp substrate drug, efflux is reduced, allowing more substrate drug to be absorbed and reach systemic circulation. This can increase the substrate drug’s bioavailability by 2-4 fold in some cases, potentially causing toxicity. Common P-gp inhibitors include verapamil (calcium channel blocker), clarithromycin and erythromycin (macrolide antibiotics), ritonavir (antiretroviral), cyclosporine (immunosuppressant), and various components in grapefruit juice. These inhibitors work by competitively blocking the P-gp binding site or by directly interfering with the transporter’s ATPase activity needed for efflux.

The classic example is the interaction between digoxin (a cardiac glycoside with narrow therapeutic index) and P-gp inhibitors. Digoxin is a P-gp substrate with bioavailability around 60-80% in most patients. When patients taking digoxin start treatment with clarithromycin or verapamil, digoxin bioavailability can increase substantially, raising plasma concentrations into the toxic range. This can lead to serious cardiac arrhythmias, nausea, visual disturbances, and confusion. The interaction develops over several days as the inhibitor accumulates, and similarly takes days to resolve after the inhibitor is discontinued. Careful monitoring of digoxin levels and dose reduction (typically by 30-50%) is necessary when P-gp inhibitors are introduced.

Conversely, P-gp inducers increase the expression and activity of P-gp, enhancing efflux and reducing substrate drug absorption. Rifampicin (rifampin), a cornerstone of tuberculosis treatment, is a potent P-gp inducer that can reduce the bioavailability of numerous drugs including digoxin, fexofenadine, and certain antiretrovirals. St. John’s Wort, a popular herbal supplement for depression, is also a significant P-gp inducer and has caused therapeutic failures with immunosuppressants, oral contraceptives, and antiretrovirals. The onset of induction is gradual (typically 7-14 days) as new P-gp protein is synthesized, and the effect persists for a similar period after the inducer is discontinued.

A short list of clinically significant P-glycoprotein substrates, inhibitors and inducers. DRUGBANK.com offers a much more comprehensive list of several hundred substrates, inhibitors and inducers

P-gp Substrates	P-gp Inhibitors	P-gp Inducers
digoxin fexofenadine dabigatran rivaroxaban apixaban loperamide colchicine	verapamil clarithromycin erythromycin ritonavir cyclosporine grapefruit juice amiodarone	rifampicin st. John’s wort carbamazepine, phenytoin, phenobarbital

The direct oral anticoagulants (DOACs)—dabigatran, rivaroxaban, and apixaban—are all P-gp substrates, and their interactions with P-gp modulators have important clinical implications. When patients on DOACs are prescribed P-gp inhibitors, the increased anticoagulant effect can lead to serious bleeding complications. Conversely, P-gp inducers can reduce DOAC levels below the therapeutic range, increasing risk of stroke or thromboembolism. Many prescribing guidelines now recommend avoiding certain P-gp inhibitors in patients taking DOACs or using alternative anticoagulants that are not P-gp substrates.

GI Physiology Interactions

Drugs that alter gastrointestinal physiology—such as motility, pH, or secretions—can indirectly affect the absorption of co-administered drugs. These interactions are particularly important because they can affect multiple drugs simultaneously and the effects may not be immediately obvious to prescribers or patients.

Motility Enhancers and Inhibitors

As discussed in Factors Affecting Drug Absorption, gastric emptying time and intestinal motility significantly influence drug absorption. Drugs that alter GI motility can therefore affect the absorption of other drugs taken concurrently.

Prokinetic agents such as metoclopramide and domperidone accelerate gastric emptying and increase intestinal motility. By hastening the transit of drugs to the small intestine (the primary absorption site), these agents can increase the rate of absorption, resulting in higher peak concentrations (c_max) and shorter time to peak (t_max). This can be clinically useful—for example, metoclopramide is sometimes co-administered with analgesics to speed pain relief in migraine treatment. However, for drugs with slow dissolution rates or those absorbed primarily in the distal small intestine, increased motility may reduce the total amount absorbed (lower AUC) by decreasing contact time with the absorptive surface.

Conversely, drugs that slow GI motility can delay absorption. Opioid analgesics are particularly problematic in this regard. Morphine, codeine, and other opioids activate μ-opioid receptors in the GI tract, causing decreased gastric emptying and reduced intestinal peristalsis. This can delay the absorption of other drugs, leading to delayed therapeutic effects. In the case of pain management, the opioid’s own absorption may also be delayed, creating a slower onset of analgesia. Chronic opioid use can lead to opioid-induced constipation, which further impairs drug absorption and can cause significant discomfort for patients.

Anticholinergic drugs (such as tricyclic antidepressants, antihistamines like diphenhydramine, and antispasmodics like oxybutynin) also reduce GI motility by blocking muscarinic receptors. The combination of multiple anticholinergic medications—common in elderly patients—can produce severe constipation and unpredictable drug absorption. Additionally, the reduced motility may increase drug contact time with intestinal metabolizing enzymes, potentially increasing first-pass metabolism for some drugs.

Dietary components significantly influence GI motility and transit time. High-fat meals are particularly important clinically as they delay gastric emptying through stimulation of cholecystokinin (CCK) release and activation of the "ileal brake" mechanism. This prolonged gastric residence time can increase the absorption of poorly soluble drugs by allowing more time for dissolution, but may also increase degradation of acid-labile drugs in the stomach. Conversely, high-fat meals accelerate small intestinal transit, potentially reducing the contact time between drugs and absorptive surfaces. High-protein meals also slow gastric emptying, though generally to a lesser degree than fats. Dietary fiber, particularly insoluble fiber, can increase intestinal transit rate and reduce drug absorption by physically trapping drugs and limiting contact with the intestinal epithelium. The clinical significance of food effects on motility is drug-specific—some medications must be taken with food to optimize absorption (eg griseofulvin), while others should be taken on an empty stomach to minimize variability (eg bisphosphonates).

Acid Modulators

Gastric pH plays a critical role in drug dissolution and ionization state, as discussed in Factors Affecting Drug Absorption. Drugs that alter gastric pH—including antacids, H₂-receptor antagonists, and proton pump inhibitors (PPIs)—can significantly affect the absorption of pH-sensitive drugs.

Antacids containing aluminium, magnesium, or calcium hydroxides rapidly neutralize stomach acid, raising gastric pH from ~1.5-3.0 to ~5.0-7.0 within minutes. This pH change can have multiple effects on drug absorption. For weakly basic drugs (such as ketoconazole and itraconazole antifungals), the less acidic environment reduces drug dissolution because these drugs require an acidic pH to convert from their insoluble base form to their soluble salt form. Ketoconazole bioavailability can decrease by 80-95% when taken with antacids, leading to treatment failure in fungal infections. The interaction can be partially managed by separating administration times (antacids 2 hours before or after ketoconazole) or by having patients take ketoconazole with acidic beverages like cola.

H₂-receptor antagonists (ranitidine, famotidine, cimetidine) produce a more sustained but less dramatic increase in gastric pH compared to antacids, typically raising pH to ~3.0-5.0. These agents can impair absorption of drugs requiring acidic conditions for dissolution, including ketoconazole, itraconazole, atazanavir (an HIV protease inhibitor), and certain tyrosine kinase inhibitors used in cancer treatment. The interaction with atazanavir is particularly problematic because adequate drug levels are essential for viral suppression. H₂ antagonists can reduce atazanavir exposure by 70-90%, potentially leading to virological failure and resistance development.

Proton pump inhibitors (omeprazole, lansoprazole, esomeprazole, pantoprazole) produce the most profound and sustained elevation of gastric pH, often maintaining pH >4.0 throughout the day. PPIs are the most widely prescribed medications worldwide, making their interaction potential highly clinically relevant. Beyond affecting the same drugs impaired by H₂ antagonists, PPIs can also reduce the absorption of nutrients like vitamin B₁₂, iron, magnesium, and calcium, potentially leading to deficiencies with long-term use. The interaction between PPIs and clopidogrel (an antiplatelet drug) has been particularly controversial—some PPIs inhibit CYP2C19, the enzyme that converts clopidogrel to its active form, potentially reducing its cardioprotective effects.

Conversely, increased gastric acidity can enhance the absorption of weakly basic drugs. Acidic beverages and ascorbic acid (vitamin C) supplements can slightly improve the absorption of ketoconazole and itraconazole by maintaining a more acidic gastric environment. Some clinicians recommend taking these antifungals with acidic drinks, though the effect is modest compared to the large impairment caused by acid suppressants.

Effects of acid-suppressing agents on drug absorption.

Drug	Mechanism of Reduced Absorption	Bioavailability Reduction with PPIs	Management Strategy
Ketoconazole	Reduced dissolution in less acidic pH	80–95%	Avoid PPIs; take with acidic beverage; use alternative antifungal
Itraconazole	Reduced dissolution in less acidic pH	60–80%	Avoid PPIs; take with acidic beverage; use alternative antifungal
Atazanavir	Reduced dissolution; pH-dependent solubility	70–90%	Avoid PPIs; use H2 antagonist with dose separation; consider alternative antiretroviral
Dasatinib	Reduced dissolution in less acidic pH	40–60%	Consider antacids (separate by 2 hours) instead of PPIs
Iron salts	Reduced conversion to absorbable ferrous form	40–70%	Take with vitamin C; avoid PPIs if possible

Dietary components also modulate gastric pH, though generally to a lesser extent than pharmaceutical acid modulators. Protein-rich meals stimulate gastric acid secretion through multiple mechanisms: gastrin release triggered by amino acids (particularly phenylalanine and tryptophan), direct stimulation of parietal cells, and calcium-mediated pathways. This increased acidity can enhance the dissolution and absorption of weakly basic drugs like ketoconazole when taken with high-protein foods. Conversely, the buffering capacity of dietary proteins can transiently neutralize stomach acid immediately after meal consumption, creating a brief period of elevated pH that may impair absorption of acid-dependent drugs. Certain foods have particularly strong buffering effects—milk and dairy products, for instance, can raise gastric pH to 5.0-6.0 for 30-60 minutes due to their protein and calcium content. This is why medications requiring acidic conditions for absorption (such as ketoconazole or certain HIV protease inhibitors) should generally be taken on an empty stomach or avoided with dairy products. Conversely, acidic foods and beverages (citrus juices, vinegar, cola drinks) maintain or lower gastric pH and may improve the dissolution of pH-sensitive basic drugs, though the clinical significance is typically modest and variable.

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