The most common reactions to oral penicillin are gastrointestinal effects and hypersensitivity reactions. Although hypersensitivity reactions have been reported much less frequently after oral than after parenteral therapy, it should be remembered that all forms of hypersensitivity, including fatal anaphylaxis have been observed with oral penicillin.
Blood and lymphatic disorders:
There have been very rare reports of changes in blood counts, including, thrombocytopenia, neutropenia, leucopenia, eosinophilia and haemolytic anaemia. Coagulation disorders (including prolongation of bleeding time and defective platelet function) have also been reported.
Nausea, vomiting, abdominal pain, diarrhoea are common. Sore mouth and black hairy tongue (discolouration of tongue) has been reported occasionally.
Hepatitis and cholestatic jaundice have been reported very rarely.
Allergic reactions may commonly occur and typically manifest as skin reactions (see Skin and subcutaneous disorders). Severe allergic reactions causing angioedema, laryngeal oedema and anaphylaxis have been reported rarely.
Serum sickness-like reactions are characterised by fever, chills, arthralgia and oedema.
Infections and infestations:
Pseudomembranous colitis has occasionally been reported.
Nervous system disorders:
Central nervous system toxicity including convulsions has been reported (especially with high doses or in severe renal impairment); paraesthesia may occur with prolonged use.
Neuropathy is an infrequent reaction and is usually associated with high doses of parenteral penicillin.
Renal and urinary disorders:
Interstitial nephritis has occurred in very rare cases.
Nephropathy is an infrequent reaction and is usually associated with high doses of parenteral penicillin.
Skin and subcutaneous disorders:
Urticarial, erythematous or mobilliform rash and pruritus occur most frequently, while exfoliative dermatitis occurs rarely.