Omnipen - Precautions | Use | Side Effects

This article was medically reviewed by M.Pharm, Marko Tanaskovic on August 12, 2018. To read more about an author, click here.

Omnipen contains active substance called Ampicillin. It leads to bacteria cell lysis which is why it is used to treat following infections:

  • Kidney infections
  • Bladder infections
  • Ureter infections
  • Urethra infections
  • Infections of the sinuses, throat, airways and lungs


Omnipen may cause prolongation of bleeding time if used in high doses, which is why it should not be used in patients with bleeding or blood clotting disorders.

Omnipen may cause decrease in concentration of neuthrophils in the blood (neuthrophils are the type of white blood cells). Neuthrophils are very important for your immune system and their drop may weaken your immune system. Go to see your doctor if you are experiencing flu-like symptoms.

You must not use Omnipen if you are allergic to any of following medications:

  • Oxacillin
  • Dicloxacillin
  • Carbenicillin
  • Penicillin
  • Amoxicillin
  • Co-amoxiclav
  • Cephalosporin antibiotics

Omnipen may lead to a life-threatening anaphylactic reaction. This adverse effect occur very rarely. Be aware of allergy symptoms (rash, itching, redness of the skin, swelling of the lips, swelling of the throat and difficulty breathing).

Check more precaution information in this post: Ampicillin.

Use of Omnipen during pregnancy and breastfeeding

Omnipen may cause slightly increased risk of cleft palate. Seek medical advice, before you start taking this medicine.

It passes in breast milk in small amounts, and it should not be used during breastfeeding.

How to use

Recommended dosage for patients above the age of 18:

Type of infection Recommended dosage
Bacterial meningitis 200 mg/kg per day divided in 6 doses
Gonorrhea One dose of 3500 mg together with Probenecid
Septicemia 200 mg/kg per day
Gastroenteritis 500 mg four times a day
Intraabdominal infections 500 mg four times a day
Skin infections 500 mg four times a day
Soft tissue infections 500 mg four times a day

Please keep in mind that this table provides only usual recommended doses. However, you cannot determine the dose that works best for you on your own. Only a doctor can determine the dose.

Since food affects its efficacy, it is recommended to take this medicine on an empty stomach.

Use with other medicines (Interactions)

Omnipen should be avoided with following medicines:

  • Lorazepam
  • Atorvastatin
  • Furosemide
  • Montelukast
  • Sulfamethoxazole/trimethoprim (Bactrim)
  • Metronidazole
  • Methotrexate

Please keep in mind that above mentioned list is not complete. You need to tell your doctor about all medicines you are taking.

Side effects

Omnipen may cause following side effects:

  • Headache
  • Dizziness
  • Seizures
  • Inflammation of the liver
  • Erythema multiforme
  • Allergy
  • Diarrhea
  • Colitis
  • Fever
  • Dermatitis
  • Pancreatitis
  • Anorexia

This list is not complete. Contact your doctor if you are experiencing any side effects.

Information on this website are provided for educational purposes only and are not intended for medical advice, diagnosis or treatment.

If you have any concerns or questions about your health, you should always consult with a physician or other health-care professional.

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