Drug Interactions

Several sites show interactions among all drugs and vitamins you type in, while they track you.

I tested them with 6 medicines which do interact: aspirin, atorvastatin, epinephrine, erythromycin, vitamin K, warfarin. ​

You can find more about drugs, including side effects, on the main drugs page.

SITES​

Drugs.com gives 5 concise warnings for these 6 drugs, in order of seriousness and 3 food interactions. If you click the professional tab, each warning is very detailed, and cites studies. It did not let me save the list, though it has a save button. (​supported by ads and trading your data with 9 trackers)

​EXAMPLE TEXT

Using warfarin together with erythromycin may cause you to bleed more easily. You may need a dose adjustment based on your prothrombin time or International Normalized Ratio (INR)
The Professional tab gives 15 citations and 380 words of explanation, including:
Coadministration with clarithromycin or erythromycin may infrequently but substantially enhance the hypoprothrombinemic effect of warfarin and other coumarin anticoagulants. The exact mechanism of interaction is unknown... In 12 normal subjects, the clearance of warfarin (1 mg/kg single dose) decreased by an average of 14% following pretreatment with erythromycin 250 mg four times a day for 8 days. In a study of eight patients...

​WebMD gives 11 concise warnings in order of seriousness, and I found no way to save the list. It owns Medscape and RxList. (supported by ads and trading your data with 17 trackers)

erythromycin oral increases effects of warfarin oral by slowing drug metabolism

RxList gives 12 warnings from WebMD, not in order of seriousness, and cannot save the list. (supported by ads and trading your data with 11 trackers)

​erythromycin oral increases effects of warfarin oral by slowing drug metabolism. ​

Medscape gives 11 concise warnings in order of seriousness, with more medical language than WebMD, but not as much as Drugs.com. (supported by ads and trading your data with 17 trackers)

erythromycin base will increase the level or effect of warfarin by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Avoid or Use Alternate Drug

CVS lets you sign in and load prescriptions or type them in. Adding prescriptions is slow, since they offer every dosage. I also worry that if you sign in to download prescriptions, and add any to evaluate, they could add these to their records and cause confusion later, even if they don't have prescriptions on file for them. (supported by sales and trading your data with 16 trackers)

These drugs may be taken together only under close supervision from your prescriber. Taking these drugs together may increase your risk to bruise or bleed. Call your doctor or health care professional if you notice any unusual bleeding. Signs of bleeding may include bloody or black, tarry stools; red or dark-brown urine; spitting up blood or brown material that looks like coffee grounds; red spots on the skin; unusual bruising or bleeding from the eye, gums, or nose. (The button to get a report did not work for me in any browser, so I got this from someone else.)

​Drug-interactions.eu charges 88 euros per year to show drug interactions. It shows side effects for free. It is managed by a professor at the University of Modena and Reggio, Italy, who seems not to track you. (supported by subscriptions with 0 trackers)

text, blood levels, side effects and 39 cites for erythromycin, 133 for warfarin
...in 12 normal subjects who took a single 1 mg/kg dose of warfarin with and without erythromycin. Erythromycin (250 mg p.o.) every 6 h for 8 days decreased warfarin clearance by 14% (p less than 0.001). Warfarin's apparent volume of distribution was not affected. Further, the effect of erythromycin was greatest among subjects whose control phase warfarin clearance was relatively slow... consistent with the interpretation that erythromycin can potentiate warfarin-induced hypoprothrombinemia by slowing warfarin clearance. Another work studied eight noninfected patients ... The plasma concentrations of warfarin and its anticoagulant effect were increased when it was co-administered with erythromycin...

UpToDate.com from Wolters Kluwer ($20/week, $45/month) covers medical and surgical treatments as well as drugs, detailed, many citations, recommendations for most conditions, drug interactions, side effects. Regular articles in UpToDate are updated every few months. The example at right of a drug interaction article is not dated, and its latest reference is from 2009. (supported by subscriptions, with 1 tracker)

19 citations and 410 words, including:
Warfarin clearance was decreased approximately 14% in healthy subjects when administered following an 8-day course of erythromycin (1 g/day).1 Other studies confirm this relatively modest impact of erythromycin on warfarin pharmacokinetics.2,3,4 Case reports, however, describe significant episodes of bleeding (hematuria, bruising) and increased prothrombin times in association with erythromycin therapy.5,6,7,8,9...