As with all aminoglycosides, at critical levels gentamicin exhibits toxicity. Nephrotoxicity may occur, resulting in a gradual reduction in creatinine clearance after about two weeks of treatment. This is rapidly reversed if the drug is withdrawn. Most cases have pre-existing renal disease or have received other nephrotoxic agents. With gentamicin the vestibular mechanism may be affected when serum levels of 10 mcg/mL or when a trough level of 2 mcg/mL are exceeded. This is usually reversible if observed promptly and the dose adjusted. In the patient with normal renal function it is virtually impossible to achieve these levels at standard dosage.