Can Ampicillin Be Administered Intramuscularly?
Yes, ampicillin can be safely administered intramuscularly (IM) and is explicitly approved for this route by the FDA for multiple indications. 1, 2
FDA-Approved IM Administration
The FDA drug label clearly states that ampicillin may be given by either the intramuscular or intravenous route for most infections. 1, 2 The standard IM preparation involves dissolving ampicillin in Sterile Water for Injection or Bacteriostatic Water for Injection to achieve a concentration of 250 mg/mL. 1, 2
Specific Dosing for IM Route
Respiratory tract infections, soft tissue infections, and GI/GU infections: Doses ranging from 250-500 mg every 6 hours can be given IM or IV. 1, 2
Bacterial meningitis: 150-200 mg/kg/day in divided doses every 3-4 hours, with treatment that may be initiated with IV drip therapy and continued with IM injections. 1, 2
Septicemia: 150-200 mg/kg/day, starting with IV administration for at least three days, then continuing with IM route every 3-4 hours. 1, 2
Gonorrheal urethritis: Two doses of 500 mg each at 8-12 hour intervals via IM or IV route. 1, 2
Clinical Guideline Support
Multiple clinical guidelines explicitly recommend IM ampicillin in specific scenarios:
Endocarditis prophylaxis: The ACC/AHA guidelines recommend ampicillin 2.0 g IM or IV (50 mg/kg IM or IV in children) within 30 minutes before dental, oral, respiratory tract, or esophageal procedures when patients are unable to take oral medication. 3
GU/GI procedures: For high-risk patients, ampicillin 2.0 g IM/IV plus gentamicin is recommended for endocarditis prophylaxis. 3
Dental procedures in renal failure patients: The AHA recommends ampicillin IM or IV for patients allergic to amoxicillin or unable to take oral medications. 3
Perioperative prophylaxis in animal models: Ampicillin 100 mg/kg IM immediately prior to surgery (2 minutes before) is suggested in research protocols. 3
Important Caveats
Stability and Timing
- Freshly prepared solutions must be used: IM injections should be administered within one hour after preparation, as potency may decrease significantly after this period. 1, 2
Pain Considerations
- IM injections can be painful: Pain at the injection site occurs in approximately one-third of patients receiving parenteral penicillins. 4
- The pain can be minimized by using lidocaine or procaine hydrochloride as diluents. 4
Route Preference in Specific Situations
- For perinatal GBS prophylaxis: The CDC guidelines specifically state that intravenous administration is the only recommended route for intrapartum chemoprophylaxis, regardless of the antimicrobial agent used, because IV achieves higher intraamniotic concentrations than IM. 3
Neonatal Considerations
- Bacteriostatic Water for Injection should not be used as a diluent when ampicillin will be used in newborns. 1, 2
Practical Administration Details
The FDA label provides specific reconstitution instructions for IM use:
- 250 mg vial: Add 1 mL diluent → 1 mL withdrawable volume at 250 mg/mL 1, 2
- 500 mg vial: Add 1.8 mL diluent → 2 mL withdrawable volume at 250 mg/mL 1, 2
- 1 gram vial: Add 3.5 mL diluent → 4 mL withdrawable volume at 250 mg/mL 1, 2
- 2 gram vial: Add 6.8 mL diluent → 8 mL withdrawable volume at 250 mg/mL 1, 2
While the 1 g and 2 g vials are primarily intended for IV use, they may be administered IM when smaller vials are unavailable. 1, 2