| Alcohol | CNS depressants Acetaminophen | Additive CNS depression, sedation, ataxia, increased risk of accidents 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 Prazosin | Masking of symptoms of hypoglycemia 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 | Risk of toxicity due to inhibition of metabolism |
| 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 | Risk of toxicity due to inhibition of metabolism |
| Ketoconazole and other azoles | Benzodiazepines, cisapride cyclosporine, fluoxetine, lovastatin, omeprazole, quinidine, tolbutamide, warfarin | Risk of toxicity due to inhibition of metabolism |
| MAO inhibitors | Catecholamine releasers (amphetamine, ephedrine) Tyramine-containing foods and beverages | Increased norepinephrine in sympathetic nerve endings released by the interacting drugs Hypertensive crisis |
| NSAIDs | Anticoagulants Angiotensin-converting enzyme (ACE) inhibitors Loop diuretics, thiazides | Increased bleeding tendency because of reduced platelet aggregation Decreased antihypertensive efficacy of ACE inhibitor Reduced diuretic efficacy |
| Phenytoin | Doxycycline, methadone, quinidine, steroids, verapamil | Reduced effect of other drugs because of induction of metabolism |
| Rifampin | Azole antifungal drugs, corticosteroids, methadone, sulfonylureas | Reduced effect of other drugs because of induction of metabolism |
| Ritonavir | Benzodiazepines, cyclosporine, diltiazem, HMG-CoA reductase inhibitors, lidocaine, metoprolol, other HIV protease inhibitors, SSRIs | Risk of ... |