Cloxacillin is an antibiotic used to treat bacterial infections caused by staphylococci, streptococci, and pneumococci. Its main role is fighting staph infections that have become resistant to standard penicillin, making it a go-to choice for skin infections, bone infections, joint infections, and bloodstream infections caused by these bacteria.
Why Cloxacillin Exists
Regular penicillin works well against many bacteria, but some strains of Staphylococcus aureus learned to fight back. These bacteria produce an enzyme called penicillinase that breaks open penicillin’s core structure, rendering it useless. Cloxacillin was designed specifically to survive this attack. Its chemical structure includes a protective side chain that physically blocks the enzyme from reaching and destroying the active part of the drug.
This means cloxacillin can kill staph bacteria that would shrug off ordinary penicillin. It works by interfering with how bacteria build their cell walls. Without intact cell walls, the bacteria die. This makes cloxacillin bactericidal, meaning it kills bacteria outright rather than simply slowing their growth.
Infections Cloxacillin Treats
Cloxacillin is prescribed for a range of infections, nearly all involving gram-positive bacteria:
- Skin and soft tissue infections: Cellulitis, erysipelas (a type of skin infection involving the upper layers), and lymphangitis (infection spreading along lymph vessels). These are among the most common reasons cloxacillin is prescribed.
- Bone infections (osteomyelitis): Staph bacteria are the leading cause of bone infections, and cloxacillin is a standard treatment. Courses for chronic osteomyelitis typically last around eight weeks, sometimes starting with intravenous treatment before switching to oral doses.
- Joint infections: Bacterial infections in joints, including those associated with prosthetic implants, are treated with cloxacillin when staph is the culprit.
- Bloodstream infections (bacteremia/sepsis): When staph bacteria enter the blood, cloxacillin is a primary treatment option, sometimes combined with other antibiotics.
It can also be combined with another antibiotic to treat urinary tract infections caused by certain gram-negative bacteria, though this is a less common use.
MSSA vs. MRSA: An Important Distinction
Cloxacillin is highly effective against methicillin-susceptible Staphylococcus aureus (MSSA), with very low concentrations needed to kill the bacteria. It also works against penicillinase-producing strains of Staphylococcus epidermidis, another common skin bacterium.
However, cloxacillin does not work against MRSA (methicillin-resistant Staphylococcus aureus) when used alone. MRSA has a different resistance mechanism that cloxacillin cannot overcome. Interestingly, research has shown that cloxacillin combined with other antibiotics can play a supporting role in treating MRSA bloodstream infections, where it helps boost the effectiveness of the partner drug and may prevent resistance from developing. But as a standalone treatment, cloxacillin is reserved for MSSA infections.
How to Take Cloxacillin
Cloxacillin and its close relatives are best absorbed on an empty stomach. Eating before taking the medication can reduce peak blood levels by 25 to 40 percent and delay the time it takes for the drug to reach its highest concentration, from about one hour to two or three hours. The standard recommendation is to take it about an hour before meals.
That said, the practical reality matters. Missing doses because you can’t coordinate them around meals may be worse than the modest reduction in absorption from taking the drug with food. If sticking to a strict fasting schedule causes you to skip doses, taking it with food is a reasonable alternative for milder infections. For serious infections with less susceptible bacteria, the timing becomes more important because you need every bit of drug concentration working in your favor.
People with kidney problems generally do not need a dose adjustment, which simplifies treatment planning.
Side Effects
The most common side effects are digestive: abdominal pain, nausea, and diarrhea. Skin rashes can also occur. These are typical of penicillin-type antibiotics and are usually mild.
Because cloxacillin belongs to the penicillin family, anyone with a known penicillin allergy should not take it. Cross-reactivity between penicillins is well established, and patients who have been recently sensitized to one penicillin are more likely to have antibodies that react to others in the same class.
A rare but serious side effect is agranulocytosis, a dangerous drop in white blood cells that weakens the immune system. This has been reported with various penicillin-type antibiotics, not just cloxacillin, and is uncommon enough that most people will never experience it.
Safety During Breastfeeding
Cloxacillin passes into breast milk in very small amounts. After a 500 mg dose, milk levels ranged from 0.1 to 0.3 mg/L, which is low enough that adverse effects in breastfed infants are not expected. In a follow-up study of ten nursing mothers taking cloxacillin, two reported diarrhea in their infants, but no rashes or oral thrush were observed. Cloxacillin is generally considered acceptable for use while breastfeeding, though minor disruption to an infant’s gut bacteria is possible with any penicillin.