An idiosyncratic reaction of unilateral common peroneal nerve palsy associated with below desired therapeutic range of tacrolimus level in a patient postheart transplantation.

Tacrolimus (TAC) is a very effective medication in routine use after solid organ transplantation. The potential, but infrequently reported neurological adverse effect of TAC is peripheral neuropathy (PN). This has rarely been reported in heart transplant patients. To the best of our knowledge, the data regarding mononeuropathy of common peroneal nerve presented with foot drop due to low whole blood trough TAC level are very limited in the early days postheart transplantation. An idiosyncratic reaction might be suspected in the early postoperative period, when the whole blood trough levels of TAC fall below or within the desired therapeutic range associated with any adverse events after ruling out other causes. We report a 21-year-old patient, who underwent heart transplantation after a suitable donor was identified, and presented with a new-onset right side foot drop on the 10th postoperative day. According to the WHO-Uppsala Monitoring Center causality assessment scale, the likely culprit agent is TAC. Rapid and progressive improvement of foot drop occurred after stopping it and changed over to cyclosporine.