What Causes Diarrhea While On Flagyl And Doxycycline?

Posted on Mon, 31 Oct 2016

Medically reviewed by Ask A Doctor - 24x7 Medical Review Team

Mon, 31 Oct 2016 Answered on

Mon, 21 Nov 2016 Last reviewed on

Question : I have been on flagyl and doxicycline for 3 1/2 days for h pylori, here recently i had 3 dark lose bowel movements and this last time that i wiped i noticed a spot of blood.
What could this be? Do these anitbiotics cause this? What do you reccomend?

Answered by Dr. Mohammed Taher Ali (6 hours later)

Brief Answer:
Suggestive of bleeding.

Detailed Answer:
Dear Madam, Hi & Welcome.
I can understand your concerns.

Stool that is dark or tarry may be an indication of bleeding in the upper part of the gastrointestinal tract. The most common medical conditions that cause dark, tar-like stool includes duodenal or stomach ulcer, gastritis, esophageal varices, & Mallory-Weiss tear (which can be linked to alcoholism).

Gastritis or stomach ulcer caused by certain medications like non-steroidal anti-inflammatory drugs can also dark stools.

Based on the history given in your case, bleeding could be due to stomach or duodenal ulcer.

The drugs (Flagyl & Doxycycline) used to eradicate Helicobater pylori infection
are unlikely to be the cause of such type of bleeding.

I would advise you to consult a surgeon or gasteroenterologist immediately for further evaluation and certain lab test like gastroscopy to identify and treat the cause of bleeding.

Hope this answers your query.
Do you have any clarification please feel free to ask.
Best regards/-

Above answer was peer-reviewed by : Dr. Nagamani Ng