1. Cardiovascular side-effects. The common cardiovascular side-effects include an increase or decrease in heart rate, irregularities in the heart rhythm, heart failure and cardiac arrest. What other drugs will affect metronidazole? Sometimes it is not safe to use certain medications at the same time. Some drugs can affect your blood levels of other drugs you take, which may increase side effects or make the medications less effective. Uses. Protozoal infections: extra-intestinal amebiasis (including amoebic liver abscess), intestinal amoebiasis (amoebic dysentery), trichomoniasis, balantidiasis. Metronidazole, marketed under the brand name Flagyl among others, is an antibiotic and antiprotozoal medication. It is used either alone or with other antibiotics to treat pelvic inflammatory disease, endocarditis, and bacterial vaginosis. It is effective for dracunculiasis, giardiasis, trichomoniasis, and amebiasis. It is an option for a first. Nausea, vomiting, loss of appetite, and allergic skin reactions are the most common side effects reported with Bactrim. Some of the problems that are less common (but are potentially serious) include watery or bloody diarrhea, shortness of breath, and decreased urination. While metronidazole is meant to remedy a dog's gastrointestinal system, it can also cause adverse side effects, including vomiting, nausea, and loss of appetite. Flagyl (metronidazole) is an antibiotic prescribed to treat various parasitic and bacterial infections (Giardia, C. diff, H. pylori). Common side effects are headaches, nausea, and metallic taste in the mouth. Pregnancy and breastfeeding safety information are provided. Metronidazole is used to treat a certain type of vaginal infection (bacterial vaginosis) in women who are not pregnant. Learn about side effects, drug interactions, dosages, warnings, and more. To reduce the development of drug-resistant bacteria and maintain the effectiveness of Metronidazole tablets and other antibacterial drugs, Metronidazole tablets should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria. Learn about Flagyl (Metronidazole) may treat, uses, dosage, side effects, drug interactions, warnings, patient labeling, reviews, and related medications.