Penicillin G Benzathine Injection

Penicillin G benzathine injection is a long-acting antibiotic active against specific bacteria sensitive to penicillin G. Bicillin L-A is the brand used for all the following:

• Strep throat and upper respiratory infections: Bicillin L-A treats mild-to-moderate infections of the upper respiratory tract caused by susceptible streptococci
• Syphilis: Bicillin L-A treats primary, secondary, latent, and late (tertiary/neurosyphilis) syphilis, as well as congenital syphilis in children
• Yaws, bejel, and pinta: Bicillin L-A treats these tropical bacterial infections
• Prevention of rheumatic fever: Bicillin L-A is used on an ongoing basis to prevent recurrence of rheumatic fever and Sydenham's chorea (a movement disorder related to strep infection)
• Preventive follow-up therapy: Bicillin L-A is also used as follow-up prophylaxis (preventive treatment) for rheumatic heart disease and acute glomerulonephritis (kidney inflammation triggered by strep)

Penicillin G kills bacteria by blocking their ability to build cell walls. Bacteria need a strong cell wall to survive — without it, they become structurally unstable and die. This only works against bacteria that are actively growing and that are susceptible to penicillin G. Because Bicillin L-A dissolves very slowly from the injection site in muscle tissue, it releases penicillin G into the blood gradually over days to weeks — giving sustained levels that other penicillin shots cannot match.

Penicillin does not work against bacteria that produce penicillinase (an enzyme that destroys penicillin) or against bacteria that have altered the proteins penicillin normally targets. Resistance to penicillin G has not been reported in Streptococcus pyogenes, the bacteria responsible for strep throat and rheumatic fever.

Bicillin L-A is given as a single deep intramuscular injection for most infections; for some stages of syphilis, a series of injections spaced a week apart is used. For long-term prevention of rheumatic fever, it is typically given once a month or every two weeks on an ongoing basis. The number of injections and how often you receive them depend on what condition is being treated — follow your prescriber's plan exactly. There is no oral (by mouth) form of this medication.

Like all antibiotics, penicillin G benzathine can cause side effects. The most commonly reported ones include:

Serious reactions — including severe allergic reactions (anaphylaxis), dangerous skin blistering (Stevens-Johnson syndrome), and C. difficile intestinal infection — require immediate medical attention. If you develop difficulty breathing, widespread skin peeling, or severe diarrhea after this injection, get emergency help right away.

• Never given IV: Bicillin L-A must only be injected into a muscle. Accidental injection into a vein has caused cardiac arrest and death.
• Injection site dangers: Injecting into or near an artery or nerve can cause permanent nerve damage, paralysis, gangrene, or tissue death — most commonly reported in infants and young children.
• Severe allergic reactions (anaphylaxis): Rare but life-threatening reactions can occur, even without a prior penicillin allergy. Emergency treatment with epinephrine may be needed.
• Severe skin reactions: Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), DRESS, and AGEP — conditions involving serious skin and organ damage — have been reported. Stop the medication immediately if widespread blistering or skin peeling develops.
• C. difficile diarrhea: Antibiotic use can disrupt gut bacteria and trigger a serious intestinal infection. Symptoms can appear up to two months after the injection. Severe or bloody diarrhea should be evaluated promptly.
• Jarisch-Herxheimer reaction: In people being treated for syphilis, a temporary flu-like reaction (chills, fever, muscle aches) can occur shortly after treatment — this is a known response to dying bacteria, not an allergic reaction.

Penicillin G benzathine has a small number of known drug interactions:

• Tetracycline antibiotics — tetracyclines slow bacterial growth rather than killing bacteria outright, which can counteract how penicillin works; these two antibiotic types should not be used together
• Probenecid (used to treat gout) — slows the kidneys' removal of penicillin, resulting in higher and longer-lasting penicillin levels in the blood; your doctor will need to factor this in if you take both

Not a complete list. The label covers the interactions the manufacturer studied or reported, and our own interaction database does not cover every medicine either. Bring your full medication list, including vitamins and herbal products, to your pharmacist.

• Tell your doctor if you have ever had an allergic reaction to any penicillin or cephalosporin antibiotic
• Tell your doctor if you have a history of allergies, asthma, hay fever, or hives — these increase the risk of a hypersensitivity reaction
• People with kidney disease clear penicillin more slowly; neonates and young infants also eliminate it more slowly — dosing and monitoring may need to be adjusted
• Inform your doctor if you are pregnant or breastfeeding before receiving this injection
• Parents of infants and young children should know that serious injection-site complications have occurred most often in this age group — only trained healthcare professionals should administer this drug
• Do not accept this medication if it is being prepared for IV (intravenous) use — it is for muscle injection only

Taking too much penicillin G can overstimulate the nervous system, leading to neuromuscular hyperirritability (unusual muscle twitching or restlessness) or convulsive seizures. If an overdose is suspected, seek emergency medical care immediately.

Penicillin G kills susceptible bacteria by preventing them from forming stable cell walls during active multiplication. Without an intact cell wall, bacteria cannot maintain their structure and die. This bactericidal (bacteria-killing) action is targeted — it is effective only against organisms that are actively growing and that have not developed resistance through penicillinase production or changes to their penicillin-binding proteins.

Because penicillin G benzathine has very low solubility, it dissolves slowly from the intramuscular injection site and is gradually converted (hydrolyzed) into active penicillin G in the body. This slow-release process produces blood levels that are lower than other penicillin shots but last far longer — detectable levels can persist for up to four weeks after a single injection. About 60% of penicillin G in the bloodstream is bound to serum proteins. The drug is distributed widely throughout body tissues, with the highest concentrations in the kidneys. It is eliminated mainly by the kidneys; clearance is slower in newborns, young infants, and people with impaired kidney function.

Store in a refrigerator, 2° to 8°C (36° to 46°F).