Penicillins: Drug-Drug Interactions
Allopurinol
The risk of rashes caused by aminopenicillins does not appear to be increased by concurrent treatment with allopurinol, as had previously been suggested.
However, aminopenicillins can still cause rash independently, and many patients receiving allopurinol have comorbid conditions that may predispose them to hypersensitivity reactions. Careful clinical observation remains important when these agents are used together.
Aminoglycosides
High doses of parenteral penicillin can inactivate aminoglycosides. In patients receiving low doses of aminoglycosides because of reduced renal function, this can be clinically important. Parenteral administration of these drugs at neonatal dosages does not appear to cause clinically relevant inactivation; therefore, separating the infusions in time is not required. Piperacillin protected against aminoglycoside nephrotoxicity without reducing aminoglycoside blood concentrations; this was possibly a protective effect of co-administered mineral salts.
When higher aminoglycoside doses are necessary, or when renal function is impaired, coordinated dosing and monitoring of serum drug levels and kidney function can help reduce the risk of underdosing or toxicity.
Cyclosporine
A study in lung transplant recipients found that nafcillin potentiated cyclosporine nephrotoxicity.
Because cyclosporine has a narrow therapeutic index and transplant patients often receive multiple nephrotoxic agents, clinicians should monitor serum creatinine and adjust immunosuppressant or antibiotic therapy if renal function worsens.
Methotrexate
Beta-lactams are weak organic acids that compete with the renal tubular secretion of methotrexate and its metabolites, reducing clearance and leading to methotrexate toxicity. Consecutive aplastic crises have been described, particularly in patients with impaired renal clearance. In contrast, co-administration of flucloxacillin in another study produced a significant, but not clinically important, reduction in methotrexate AUC.
More basic interactions between piperacillin and methotrexate, as well as its major metabolite 7-hydroxymethotrexate, have been studied in rabbits. The interaction was mainly attributed to reduced renal clearance of both methotrexate and its metabolite.
The authors concluded that renal function should be monitored in patients taking this combination, with adequate fluid intake maintained, especially in older adults, because dehydration may accelerate the onset of toxicity.
In practice, frequent assessment of kidney function, blood counts, and clinical status can help detect early signs of methotrexate accumulation, particularly in patients who are already at risk for nephrotoxicity or bone marrow suppression.
Phenytoin
Competitive albumin binding of drugs with high serum protein affinity can increase pharmacologically active unbound concentrations and enhance the metabolism of low-clearance drugs. In vitro data suggest that high doses of oxacillin significantly increase unbound phenytoin concentrations, especially in patients with hypoalbuminemia or uremia.
Because total phenytoin levels may underestimate exposure when protein binding is altered, measurement of free phenytoin levels and clinical monitoring for neurologic toxicity are advisable when high-dose oxacillin is used in patients receiving phenytoin.
| Co-administered drug | Penicillin or class | Interaction and clinical considerations |
|---|---|---|
| Allopurinol | Aminopenicillins | Earlier concerns about increased rash risk have not been confirmed. Monitor for hypersensitivity reactions, especially in patients with other risk factors. |
| Aminoglycosides | High-dose parenteral penicillins; piperacillin | High doses can inactivate aminoglycosides, which may be clinically important in patients with renal impairment. Piperacillin may reduce nephrotoxicity without lowering aminoglycoside levels; monitor drug levels and kidney function. |
| Cyclosporine | Nafcillin | Nafcillin potentiated cyclosporine nephrotoxicity in lung transplant recipients. Check creatinine regularly and reassess therapy if renal function declines. |
| Methotrexate | Beta-lactams, including piperacillin and flucloxacillin | Competition for renal tubular secretion reduces methotrexate clearance and increases the risk of toxicity, particularly in patients with renal impairment. Ensure adequate hydration and monitor laboratory parameters closely. |
| Phenytoin | High-dose oxacillin | Displacement from albumin binding increases unbound phenytoin concentrations, especially in patients with hypoalbuminemia or uremia. Consider free phenytoin levels and monitor for signs of toxicity. |
Interference with Diagnostic Tests
Pseudoproteinuria
Patients who take penicillin G or ureidopenicillin derivatives at doses over 5 g/day may develop pseudoproteinuria. Proteinuria should be evaluated using a bromphenol blue test (Albustix) or after urine dialysis.
Recognizing this effect is important to avoid misinterpreting benign test interference as evidence of intrinsic kidney disease.
17-ketosteroids
High-dose penicillin produces abnormally high concentrations of 17-ketogenic steroids in the blood and high concentrations of 17-ketosteroids in the urine.
When an endocrine evaluation is needed, clinicians should consider recent or ongoing high-dose penicillin therapy, as it can complicate the interpretation of steroid measurements.
| Test or marker | Effect described | Practical implication |
|---|---|---|
| Urine protein tests | Pseudoproteinuria with penicillin G or ureidopenicillins at doses over 5 g/day | Confirm suspected proteinuria using bromphenol blue testing or after urine dialysis to distinguish an artifact from true renal protein loss. |
| 17-ketosteroids and 17-ketogenic steroids | Abnormally high blood and urine concentrations with high-dose penicillin | Interpret endocrine workups cautiously in patients receiving high-dose penicillins, and repeat testing after therapy if necessary. |