Overview of Selected Serious Drug Interactions

From: http://www.aafp.org/afp/20000315/1745.html

Interaction
Potential effect
Time to effect
Recommendations and comments
Warfarin (Coumadin) plus ciprofloxacin (Cipro), clarithromycin (Biaxin), erythromycin, metronidazole (Flagyl) or trimethoprim-sulfamethoxazole (Bactrim, Septra)
Increased effect of warfarin
Generally within 1 week
Select alternative antibiotic.

Warfarin plus acetaminophen
Increased bleeding, increased INR
Any time
Use lowest possible acetaminophen dosage and monitor INR.

Warfarin plus acetylsalicylic acid (aspirin)
Increased bleeding, increased INR
Any time
Limit aspirin dosage to 100 mg per day and monitor INR.

Warfarin plus NSAID
Increased bleeding, increased INR
Any time
Avoid concomitant use if possible; if coadministration is necessary, use a cyclooxygenase-2 inhibitor and monitor INR.

Fluoroquinolone plus divalent/trivalent cations or sucralfate (Carafate)
Decreased absorption of fluoroquinolone
Any time
Space administration by 2 to 4 hours.

Carbamazepine (Tegretol) plus cimetidine (Tagamet), erythromycin, clarithromycin or fluconazole (Diflucan)
Increased carbamazepine levels
Generally within 1 week
Monitor carbamazepine levels.

Phenytoin (Dilantin) plus cimetidine, erythromycin, clarithromycin or fluconazole
Increased phenytoin levels
Generally within 1 week
Monitor phenytoin levels.

Phenobarbital plus cimetidine, erythromycin, clarithromycin or fluconazole
Increased phenobarbital levels
Generally within 1 week
Clinical significance has not been established.Monitor phenobarbital levels.

Phenytoin plus rifampin (Rifadin)
Decreased phenytoin levels
Generally within 1 week
Clinical significance has not been established.Monitor phenytoin levels.

Phenobarbital plus rifampin
Decreased phenobarbital levels
Generally within 1 week
Monitor phenobarbital levels.

Carbamazepine plus rifampin
Decreased carbamazepine levels
Generally within 1 week
Clinical significance has not been established.Monitor carbamazepine levels.

Lithium plus NSAID or diuretic
Increased lithium levels
Any time
Decrease lithium dosage by 50% and monitor lithium levels.

Oral contraceptive pills plus rifampin
Decreased effectiveness of oral contraception
Any time
Avoid if possible. If combination therapy is necessary, have the patient take an oral contraceptive pill with a higher estrogen content (>35 µg of ethinyl estradiol) or recommend alternative method of contraception.

Oral contraceptive pills plus antibiotics
Decreased effectiveness of oral contraception
Any time
Avoid if possible. If combination therapy is necessary, recommend use of alternative contraceptive method during cycle.

Sildenafil (Viagra) as well cialis or levitra plus nitrates
Dramatic hypotension
Soon after taking sildenafil
Absolute contraindication.

Sildenafil plus cimetidine, erythromycin, itraconazole or ketoconazole
Increased sildenafil levels
Any time
Initiate sildenafil at a 25-mg dose.

HMG-CoA reductase inhibitor plus niacin, gemfibrozil (Lopid), erythromycin or itraconazole
Possible rhabdomyolysis
Any time
Avoid if possible. If combination therapy is necessary, monitor the patient for toxicity.

Lovastatin (Mevacor) plus warfarin
Increased effect of warfarin
Any time
Monitor INR.

SSRI plus tricyclic antidepressant
Increased tricyclic antidepressant level
Any time
Monitor for anticholinergic excess and consider lower dosage of tricyclic antidepressant.

SSRI plus selegiline (Eldepryl) or nonselective monoamine oxidase inhibitor
Hypertensive crisis
Soon after initiation
Avoid.

SSRI plus tramadol (Ultram)
Increased potential for seizures; serotonin syndrome
Any time
Monitor the patient for signs and symptoms of serotonin syndrome.

SSRI plus St. John's wort
Serotonin syndrome
Any time
Avoid.

SSRI plus naratriptan (Amerge), rizatriptan (Mazalt), sumatriptan (Imitrex) or zolmitriptan (Zomig)
Serotonin syndrome
Possibly after initial dose
Avoid if possible. If combination therapy is necessary, monitor the patient for signs and symptoms of serotonin syndrome.
INR = International Normalized Ratio; NSAID = nonsteroidal anti-inflammatory drug; HMG-CoA = 3-hydroxy-3-methylglutaryl­coenzyme A reductase inhibitor; SSRI = selective serotonin reuptake inhibitor.