Doxycycline and Flagyl: Similarities, Differences, Uses, Interactions

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Doxycycline and Metronidazole (Flagyl) are potent antibiotics used to treat many infections caused by bacteria and parasites. Flagyl, or Metronidazole, is prescribed to treat bacterial infections caused by anaerobic bacteria or parasites. Similarly, Doxycycline treats many bacterial infections, including acne, respiratory, and urinary tract infections.

Doxycycline typically stops the growth of bacteria and has anti-inflammatory qualities. Doxycycline is also used under the direction of a qualified healthcare professional to prevent malaria.

This article discusses Doxycycline and Flagyl, their similarities, differences, uses, dosages, and additional information.

Understanding Doxycycline

Doxycycline is a popular antibiotic medication categorized in the tetracycline class that deters the reproduction and growth of bacteria and has anti-inflammatory qualities. It cures a wide range of bacterial infections, including respiratory and urinary tract infections, acne, gum disease, gonorrhea, chlamydia, syphilis, and eye infections. Doxycycline prevents malaria and cures infections caused by lice, ticks, or mites.

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What is Flagyl?

Flagyl is a popular antibiotic in the nitroimidazole class used to treat many infections caused by bacteria or parasites. It is also known as Metronidazole and treats BV, sexually transmitted diseases, anaerobic bacterial infections, abscesses in the liver or abdomen, colorectal surgery infection, acne rosacea, and diarrhea.

Flagyl also treats infections caused by parasites, such as trichomoniasis, Gardnerella infection, and amebiasis. Doctors also combine Flagyl with other medications to treat H. pylori infection that causes intestinal or stomach ulcers.

Flagyl is typically effective against bacteria and specific parasites in anaerobic environments. Anaerobic bacteria are single-celled microorganisms that grow in environments with minimal oxygen. These bacteria cause infections in the pelvis, abdomen, and liver. Flagyl limits synthesizing the bacteria’s DNA, stopping reproduction and growth.

Doxycycline and Flagyl: What are the side effects

Doxycycline is a typically mild antibiotic that doesn’t usually cause side effects. However, like every medication, side effects may occur sometimes. Here are some of the common and less common side effects of Doxycycline.

Doxycycline side effects

Common Side Effects:

  • Nausea and vomiting
  • Upset stomach
  • Loss of appetite
  • Mild diarrhea
  • Skin rash or itching
  • Darkened skin color
  • Vaginal itching or discharge.

Less Common Side Effects:

Sensitivity to the sun: Protecting your skin from excessive exposure while taking Doxycycline is essential.


Temporary discoloration of adult teeth: This effect usually resolves with a dentist cleaning after you stop taking the drug.

Remember that most side effects are usually mild and resolve on their own. However, in rare cases, more severe adverse effects, such as allergic reactions and liver complications, may occur. If you experience any unusual symptoms, consult your qualified healthcare provider promptly.

4 Common Side Effects of Doxycycline

Flagyl side effects

Flagyl (generic name: Metronidazole) is an antibiotic used to treat various bacterial infections. While it can be effective, being aware of potential side effects is essential. Here are some common and less common side effects associated with Flagyl:

Common Side Effects:

  • Nausea
  • Dizziness
  • Headache
  • Stomach upset
  • Vomiting
  • Loss of appetite
  • Diarrhea
  • Constipation
  • A metallic taste in your mouth
  • Darkening of urine.

Serious Side Effects:

  • Agitation
  • Blurred vision
  • Confusion
  • Depression
  • Seizures
  • Unsteadiness or trembling
  • Unusual tiredness or weakness
  • Fever
  • Eye pain
  • Changes in speech patterns
  • Seeing or hearing things that are not there
  • Slurred speech
  • Stiff neck or back
  • Trouble speaking
  • Weakness in the arms, hands, legs, or feet.

Less Common Side Effects:

  • Blood in the urine
  • Burning while urinating
  • Body aches or pain
  • Fainting
  • Irregular heartbeat
  • Loss of voice
  • Nasal congestion
  • Skin rash, hives, redness, itching
  • Stomach and back pain (severe)
  • Vaginal irritation, discharge, or dryness

Rare Side Effects:

  • Bleeding gums
  • Chest pain
  • Dark urine
  • Indigestion
  • Painful or difficult urination
  • Swollen glands
  • Yellow eyes or skin

Remember to consult your doctor if you experience severe or persistent symptoms while taking Flagyl.

Which is better for acne, Flagyl or Doxycycline?

Flagyl and Doxycycline can be used to treat acne. However, doctors recommend Doxycycline as a first-line treatment, while Flagyl isn’t.


Doxycycline is one of the first-line treatments for acne and has become more popular than tetracycline.

Dosage: It is typically started at 50 milligrams (mg) to 100 mg twice a day and should be taken with food to avoid significant nausea.

Doxycycline is more likely than tetracycline to cause photosensitivity (increased sensitivity to the sun), which can lead to sunburns.

This medication is available under various brand-names, including Doryx, Oracea, Monodox, Atridox, Morgidox, Vibra-Tabs, Alodox, Ocudox, Doxy, Acticlate, and Vibramycin.

Flagyl (Metronidazole):

Flagyl is not commonly used as a first-line treatment for acne; it is typically considered a second-line treatment. A dermatologist should decide which antibiotic to use on a case-by-case basis.

Doxycycline is generally preferred due to its efficacy and tolerability in treating acne.

Metronidazole (Flagyl) may be considered if other treatments are ineffective or if there are specific reasons to use it.

Remember that individual responses to medications can vary, and it’s essential to consult with a qualified dermatologist to determine the most suitable treatment based on your specific condition and needs. Before recommending a treatment course, they will consider statistical analysis of your acne’s severity, potential side effects, and other health considerations.

Which is better for BV, Flagyl or Doxycycline?

Bacterial vaginosis (BV) is a common condition resulting from an overgrowth of bacteria in the vagina. It can cause symptoms such as foul-smelling vaginal discharge, grayish or white discharge, burning sensation while urinating, and itching around the vagina area. Both Flagyl (Metronidazole) and Doxycycline can be used to treat BV, but they have different characteristics:

Flagyl (Metronidazole):

Flagyl is commonly used to treat BV. It is available as oral pills, a vaginal gel, or a gel for insertion into the vagina. It helps stop the overgrowth of bacteria associated with BV. The gel form is suitable for pregnant women.


Doxycycline is not commonly used as a first-line treatment for BV; it is typically considered a second-line treatment. It is usually taken orally, starting at 50 to 100 mg twice daily. Doxycycline can cause photosensitivity (increased sensitivity to the sun), but it is effective against BV-associated bacteria.

Doxycycline is generally preferred due to its efficacy and tolerability in treating BV.

Metronidazole (Flagyl) may be considered if other treatments are ineffective or if there are specific reasons to use it.

What is the difference between Flagyl and Doxycycline?

Doxycycline and Flagyl are antibiotics that treat similar bacterial infections. However, they have specific differences and peculiarities.

Flagyl (Metronidazole):

Usage: Flagyl is an antibiotic primarily used to treat various bacterial infections, including those of the vagina, stomach, liver, skin, joints, brain, and respiratory tract.

Effectiveness: It is especially effective against anaerobic bacteria and certain parasites.

Forms: Available in oral tablets, vaginal gel, and gel for vaginal insertion.

Common Uses:

  • Treatment of BV.
  • Management of intra-abdominal infections.
  • Control of amebic liver abscesses.
  • Prevention of postoperative infections.

Side Effects: Common side effects include nausea, dizziness, headache, stomach upset, vomiting, and a metallic taste in the mouth.

Precautions: Avoid alcohol while taking Flagyl and for three days after stopping it.

Serious Side Effects: These may include seizures, depression, and changes in speech patterns.

Photosensitivity: Flagyl does not cause photosensitivity.


Usage: Doxycycline is an antibiotic commonly used for treating various infections, including acne, respiratory tract infections, and sexually transmitted diseases.

Effectiveness: It is effective against many bacteria.

Forms: Available in oral capsules or tablets.

Common Uses:

  • Treatment of acne (especially when other therapies fail).
  • Management of respiratory tract infections.
  • Prevention of malaria.
  • Control of sexually transmitted infections.

Side Effects: Regular adverse effects include nausea, diarrhea, and photosensitivity.

Precautions: Take with food to avoid nausea. Be cautious about sun exposure due to photosensitivity.

Serious Side Effects may include agitation, blurred vision, and instability.

Photosensitivity: Doxycycline can cause photosensitivity.

Brand Names: Available under various brand-names, including Doryx, Oracea, Monodox, and Vibramycin.

Flagyl is commonly used for BV, while Doxycycline is preferred for acne and other infections.

Their choice depends on the specific condition, individual factors, and potential interactions with other medications.

Are Doxycycline and Metronidazole (Flagyl) safe during pregnancy or breastfeeding?

Doxycycline and Metronidazole have different safety profiles during pregnancy and breastfeeding:

Doxycycline is not recommended during pregnancy due to potential effects on tooth and bone development in the baby. Consult your doctor for alternative antibiotics.

If your baby is healthy, you can take Doxycycline for less than 3 weeks while breastfeeding.

Flagyl is generally considered safe during pregnancy and breastfeeding. As with any medication, there are potential side effects. Consult your healthcare provider before taking Metronidazole if pregnant or breastfeeding.

Doxycycline and Flagyl: Drug interaction

There are no known contraindications for Flagyl and Doxycycline. However, some drug interactions may occur when they’re combined with certain medications. To avoid unwanted complications, speaking to your medical expert before using Flagyl or Doxycycline is essential.

Flagyl interactions

Alcohol: Avoid alcohol while taking Flagyl. Combining the two can lead to severe nausea, vomiting, flushing, and headache.

Warfarin: Flagyl may increase the effects of warfarin, a medication used to prevent blood clots. Regular monitoring of blood clotting levels is essential if you are taking both drugs.

Lithium: Flagyl can increase lithium levels in the blood, potentially leading to toxicity. Close monitoring is necessary if you are on lithium therapy.

Disulfiram: Taking Flagyl with disulfiram (used to help people stay off alcohol) can cause a disulfiram-like reaction characterized by nausea, vomiting, and flushing.

Phenytoin or Phenobarbitone: Flagyl may increase the effects of these antiepileptic drugs, so monitoring is crucial.

Doxycycline interactions

Doxycycline can interact with various other medications and substances. Here are some significant interactions to be aware of:

Antacids: Avoid taking antacids containing aluminum, calcium, or magnesium concurrently with Doxycycline. These antacids can reduce the absorption of Doxycycline.

Penicillin: There is a potential interaction between Doxycycline and penicillin. It’s essential to monitor closely if both drugs are used together.

Anticoagulants (Blood Thinners): Doxycycline may interact with anticoagulants like warfarin. Regular monitoring of blood clotting levels is crucial when using both medications.

Bismuth Subsalicylate (Bismuth): Taking Doxycycline with bismuth subsalicylate (commonly found in medications like Pepto-Bismol) may reduce its effectiveness. It’s advisable to separate their administration by a few hours.

How long does it take for Doxycycline to work?

How long does it take for Doxycycline and Metronidazole (Flagyl) to work?

Flagyl and Doxycycline are potent antibiotics that treat similar bacterial infections. However, the onset of action for both medications can differ depending on the specific condition individual responses and infection being treated:


Improvement may be evident within a few days of starting treatment for some infections.

For more severe infections, it may take longer to observe significant improvement.

Completing the full course of Doxycycline prescribed by your doctor, even if you feel better, is essential to completely eradicate the infection.

Metronidazole (Flagyl):

Due to rapid absorption, Metronidazole starts to work 1 to 2 hours after ingestion.

It reaches its maximum concentration in the body within 20 minutes to 3 hours.

However, it may take a couple of days before you feel better or notice an improvement in your symptoms.

Continue taking the entire course of Metronidazole as prescribed, even if symptoms improve, to prevent recurrence.

Read also: Amoxicillin and Flagyl

Frequently Asked Questions

Doxycycline and Metronidazole can be taken together, but a qualified healthcare professional must recommend that before combining both antibiotics, there are a few points to note.

There are no known interactions between both medications.

Doxycycline and Metronidazole are prescription antibiotics prescribed to treat different types of infections. In the case of multiple infections, taking them together may be necessary. Metronidazole and Doxycycline should only be taken as directed by a certified healthcare provider.

Doxycycline, a tetracycline antibiotic, can help treat a bacterial gum infection known as periodontitis. While it is not explicitly indicated for wisdom tooth abscesses, it may be beneficial in some instances.

Doxycycline inhibits bacterial growth and is used to treat periodontitis, which includes gum infections. Although primarily for periodontitis, Doxycycline may have other applications. Always consult your healthcare expert or pharmacist if you have questions about its use.

Doctors usually recommend a combination of Doxycycline and Metronidazole to treat pelvic inflammatory disease (PID). PID is a complication that occurs from prolonged and untreated bacterial vaginosis or other sexually transmitted diseases.

Doctors typically recommend 100 mg of Doxycycline orally twice daily for 14 days. This medication eliminates the bacteria that causes PID.

For Flagyl, doctors recommend 500 mg orally twice daily for 14 days. Flagyl complements the Doxycycline course by targeting bacteria in anaerobic environments.

If you miss a Doxycycline or Flagyl (Metronidazole) dose, there’s no need to panic.

If you forget a Doxycycline dose, take it as soon as you remember.

However, if it’s almost time for your next dose, skip the missed dose and take your next dose as scheduled.

Do not skip your doses to reduce the chances of further infection or antibiotic resistance. If you miss your Flagyl dose, it is recommended that you continue taking the antibiotic even if you miss a scheduled dose.

The content on Doctor Alexa’s blog is reviewed by Advanced practice registered nurses or pharmacist and is intended for educational purposes only. This information should not be relied upon as professional medical counsel. Be sure to always consult with your physician about the dangers and benefits of any medication, treatment or procedure.

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