Meropenem × Valproic acid

Carbapenems collapse valproic acid levels. The AEMPS meropenem label gives the figure: on concomitant use the blood level of valproate falls by 60–100% within about two days, and the same label states that because of the rapid onset and the depth of the fall the combination is not considered manageable and is therefore avoided. The valproate label repeats this from its own side. The mechanism has two parts: the carbapenem inhibits acylpeptide hydrolase, which converts valproate glucuronide back into the active drug, and speeds distribution of valproate into red cells. Raising the valproate dose does not rescue it: in published cases the level did not return to the therapeutic range while the carbapenem stayed in the regimen.

Loss of seizure control 1–3 days after the first meropenem dose: fits return after months without any, up to clusters and status epilepticus. In a patient taking valproate as a mood stabiliser, mania or an abrupt mood swing appears instead of seizures. The blood valproate level drops below the therapeutic range, sometimes to undetectable, with the dose unchanged and adherence intact. Fever and the infection itself lower the seizure threshold further, so the picture is easily written off as the underlying illness.

The combination is taken apart. If meropenem is needed for a life-threatening infection, the anticonvulsant regimen is rebuilt in advance: a drug with a different mechanism is added (levetiracetam, lacosamide, a benzodiazepine for a short course) and kept through the antibiotic course and for several days after. Where the severity of the infection allows, the antibiotic is switched to a non-carbapenem: a cephalosporin with metronidazole, piperacillin-tazobactam, or a fluoroquinolone by susceptibility. Valproate levels are checked before meropenem starts, at 24–48 hours and after the antibiotic stops: recovery is not instant either, and by then the regimen needs revisiting again. Raising the valproate dose as the only measure does not work and adds toxicity risk once the antibiotic ends.