Teriflunomide, a novel, orally bioavailable, active metabolite of leflunomide, has anti-inflammatory activity. It is prescribed as a first-line treatment for relapsing–remitting multiple sclerosis (RRMS) at a dose of one 14mg tablet per day. Common adverse reactions observed in placebo-controlled trials with a frequency≥10% and a rate twofold or more than reported with placebo, include digestive disorders. As teriflunomide tablets also contain lactose, the official recommendations are clear about not prescribing this drug to patients with known lactose intolerance and those with rare hereditary problems due to galactose intolerance.

Our study systematically collected, from our MS clinical practice, all adverse events presenting in the first 100 patients treated with teriflunomide. All of these patients were systematically asked if they were known to have lactose intolerance.

None of these 100 patients declared having known, documented lactose intolerance. Yet, after starting teriflunomide, 14 reported mild-to-moderate diarrhea, which resolved within a month, but four of these patients continued to have daily diarrhea (grade 2 WHO classification), prompting us to perform a lactose breath test (LBT) for malabsorption. All four tested positive and were therefore diagnosed with lactose intolerance. Digestive symptoms were resolved with probiotics, and teriflunomide was maintained in three cases; the fourth patient decided, despite the adverse event being resolved, to stop taking teriflunomide.

In cases of prolonged digestive side-effects after the introduction of teriflunomide, a lactose-malabsorption breath test should be proposed to confirm the culpability or not of an enzymatic defect in the occurrence of adverse events.