Dicloxacillin is an antibiotic used to treat infections caused by staph bacteria, particularly skin and soft tissue infections. It belongs to a group called penicillinase-resistant penicillins, meaning it can fight staph bacteria that have learned to destroy regular penicillin. It comes as a capsule or oral suspension and is taken four times a day.
How Dicloxacillin Works
Bacteria need to build and maintain a cell wall to survive. Dicloxacillin kills bacteria by blocking the construction of that wall during active growth, causing the bacterial cell to break apart and die. What makes it special compared to regular penicillin is its resistance to an enzyme called penicillinase. Many staph bacteria produce this enzyme specifically to destroy penicillin-type antibiotics. Dicloxacillin’s chemical structure resists that attack, so it stays active where standard penicillin would fail.
Infections It Treats
Dicloxacillin is primarily used against Staphylococcus aureus infections, specifically the methicillin-sensitive type (MSSA). Its approved uses include skin and soft tissue infections, respiratory tract infections like pharyngitis and pneumonia, and streptococcal infections. Doctors also commonly prescribe it for conditions not listed on the official label, including:
- Boils and carbuncles
- Impetigo
- Cellulitis
- Folliculitis
- Hidradenitis suppurativa
- Long-term suppression of staph joint infections in people with prosthetic joints
One important limitation: dicloxacillin does not work against MRSA. MRSA bacteria carry a gene that changes the target site where all penicillin-type drugs attach, making them resistant to dicloxacillin, oxacillin, nafcillin, and cephalosporins alike. If a culture comes back showing MRSA, a different antibiotic is needed. However, doctors may start dicloxacillin before lab results are available when they suspect a staph infection that’s likely to be the sensitive type.
How to Take It
Dicloxacillin is best absorbed on an empty stomach. Take it at least one hour before or two hours after eating. For mild to moderate infections, the typical adult dose is 125 mg every six hours. Severe infections call for 250 mg every six hours, though some treatment guidelines recommend up to 500 mg four times daily for serious soft tissue staph infections.
If you’re treating a strep infection, expect to take it for at least 10 days. Cutting the course short with strep increases the risk of complications like rheumatic fever or kidney inflammation. For staph infections, the duration depends on how deep and serious the infection is.
If you have the liquid suspension form, it stays stable for about seven days at room temperature or up to 10 days in the refrigerator. Discard any leftover suspension after that window.
Common Side Effects
The most frequent side effects are digestive: diarrhea, nausea, and vomiting. Some people notice mouth irritation or a harmless but startling black, hairy appearance on the tongue, which resolves after stopping the medication. Swollen joints can also occur.
More serious reactions that need prompt medical attention include rash, hives, itching, or any swelling of the face, throat, or tongue. Difficulty breathing or wheezing after a dose signals a possible allergic reaction. Severe watery or bloody diarrhea, sometimes accompanied by stomach cramps and fever, can indicate a secondary gut infection. This complication can appear up to two months after finishing the antibiotic, so it’s worth knowing about even after your course ends.
Who Should Not Take It
Dicloxacillin is a penicillin, so anyone with a history of severe penicillin allergy should avoid it. This includes people who have experienced anaphylaxis, serious skin reactions like Stevens-Johnson syndrome, or organ damage (kidney injury, hemolytic anemia) after taking any penicillin-type drug. These individuals should avoid all beta-lactam antibiotics, the broader family that includes penicillins and cephalosporins.
People with a milder or more distant history of penicillin allergy may still need evaluation. Cross-reactivity between penicillins and newer cephalosporins is low, under 1% for third-generation types, but the risk is real enough that your prescriber needs to know your full allergy history.
Interactions With Other Medications
If you take a blood thinner like warfarin, dicloxacillin can lower its effectiveness, reducing your blood’s clotting time in the opposite direction from what you want. This interaction becomes more significant when the antibiotic course lasts longer than 14 days. Your prescriber may need to adjust your blood thinner dose or monitor your clotting levels more frequently during treatment.
As with other penicillins, dicloxacillin can also interact with certain other medications, so make sure your doctor and pharmacist know everything you’re currently taking before starting a course.