As with other β-lactam antibiotic, haematological effects including reversible leucopenia, reversible thrombocytopenia and haemolytic anaemia have been reported rarely.

Immune System Disorders:

Anaphylaxis (see Item 4.4-Warnings) has been reported rarely

If any hypersensitivity reaction occurs, the treatment should be discontinued

Late sensitivity reactions may include serum sickness-like reactions (featuring symptoms such as arthralgia, rash, urticaria, fever, angioedema, lymphadenopathy), haemolytic anaemia and acute interstitial nephritis.

Metabolism and nutrition disorders:

Electrolyte disturbances, such as hypokalaemia, due to administration of large amounts of sodium

Psychiatric disorders:

There is a potential for hallucinations to occur rarely with flucloxacillin.

Nervous System Disorders

Coma may develop with high doses of Flucloxacillin.

Respiratory, thoracic and mediastinal disorders:

Bronchospasm may occur as a result of penicillin allergy.

There is a potential for acute, severe dyspnoea to occur with flucloxacillin.

Gastrointestinal disorders:

Minor gastrointestinal disturbances, including occasionally nausea, vomiting and diarrhoea may occur during treatment. Pseudomembranous colitis has been reported rarely.

Hepatobiliary disorders:

Hepatitis and cholestatic jaundice have been reported rarely. These may be delayed for up to two months after withdrawal of treatment. In some cases the course of these conditions has been protracted and lasted for several months. 

Very rarely deaths have been reported from hepatic effects but are mostly limited to patients with serious underlying disease.

As with most other antibiotics, a moderate transient increase in transaminases has been reported.

Skin and subcutaneous tissue disorders:

Skin rash, puritis and urticaria have been reported. The incidence of rash is higher in patients suffering from infectious mononucleosis and acute or chronic leukaemia of lymphoid origin. Purpura, fever, eosinophilia and sometimes angioneurotic oedema have also been reported. Rarely, skin reactions such as erythema multiforme, Stevens-Johnson syndrome, and toxic epidermal necrolysis have been reported. Reactions such as fever, arthralgia, and myalgia can develop more than 48 hours after the start of the treatment.

Erythema nodosum may occur rarely with flucloxacillin.

Potential for pemphigoid reactions to occur rarely with flucloxacillin.

There is potential for non-thrombocytopenic purpura to occur rarely with flucloxacillin.

Vasculitis may occur rarely with flucloxacillin.

Renal and urinary disorders:

Interstitial nephritis may occur but it is reversible when treatment is discontinued.

Congenital, familial and genetic disorders: 

Potential for acute attacks of porphyria to occur with flucloxacillin.

General disorders and administration site conditions:

Some patients with spirochaete infections such as syphilis or leptospirosis may experience a Jarisch-Herxheimer reaction shortly after treatment with a penicillin is started. Symptoms include fever, chills, headache and reaction at the site of lesions. The reaction can be dangerous in cardiovascular syphilis or where there is a serious risk of increased local damage such as with optic atrophy