| Alcohol | CNS depressants | Additive CNS depression, sedation, ataxia, increased risk of accidents |
| Acetaminophen | Increased formation of hepatotoxic metabolites of acetaminophen |
| Antacids | Digoxin, iron supplements, fluoroquinolones, ketoconazole, tetracyclines, thyroxine | Decreased gut absorption due either to reaction with the affected drug or due to reduced acidity |
| Antihistamines (H1 blockers) | Antimuscarinics, sedatives | Additive effects with the drugs affected |
| Antimuscarinic drugs | Drugs absorbed from the small intestine | Slowed onset of effect because stomach emptying is delayed |
| Barbiturates, especially phenobarbital | Azoles, calcium channel blockers, cyclosporine, propranolol, protease inhibitors, quinidine, steroids, warfarin, and many other drugs metabolized in the liver | Increased clearance of the affected drugs due to enzyme induction, possibly leading to decreases in drug effectiveness |
| Beta blockers | Insulin | Masking of symptoms of hypoglycemia |
| Prazosin | Increased first-dose syncope |
| Bile acid-binding resins | Acetaminophen, digitalis, thiazides, thyroxine | Reduced absorption of the affected drug |
| Carbamazepine | Cyclosporine, doxycycline, estrogen, haloperidol, theophylline, warfarin | Reduced effect of other drugs because of induction of metabolism |
| Cimetidine | Benzodiazepines, lidocaine, phenytoin, propranolol, quinidine, theophylline, warfarin, dofetilide | Risk of toxicity due to inhibition of metabolism or reduced renal excretion |
| Disulfiram, metronidazole, certain cephalosporins | Ethanol | Increased hangover effect due to inhibition of aldehyde dehydrogenase |
| Erythromycin | Carbamazepine, cisapride, quinidine, sildenafil, SSRIs | Risk of toxicity due to inhibition of metabolism |
| Furanocoumarins (grapefruit juice) | Alprazolam, atorvastatin, cyclosporine, midazolam, nifedipine | Risk of toxicity due to inhibition of metabolism |
| Ketoconazole and other azoles | Benzodiazepines, cisapride, cyclosporine, fluoxetine, lovastatin, omeprazole, quinidine, tolbutamide, oral anticoagulants (warfarin, apixaban, rivaroxaban) | Risk of toxicity due to inhibition of metabolism, eg, increased bleeding with oral anticoagulants, myopathy with statins |
| MAO inhibitors | Catecholamine releasers (amphetamine, ephedrine) | Increased norepinephrine in sympathetic nerve endings released by the interacting drugs |
| Tyramine-containing foods and beverages | Hypertensive crisis |
| NSAIDs | Anticoagulants (warfarin, apixaban, dabigatran, rivaroxaban) | Increased bleeding tendency because of reduced platelet aggregation (additive anticoagulant effect) |
| Angiotensin-converting enzyme (ACE) inhibitors | Decreased antihypertensive efficacy of ACE inhibitor |
| Loop diuretics, thiazides | Reduced diuretic efficacy |
| Phenytoin | Doxycycline, methadone, quinidine, steroids, verapamil, anticoagulants (warfarin, apixaban, rivaroxaban) | Reduced effect of other drugs because of ... |