To compare the efficacy and safety of mebendazole and secnidazole in the treatment of giardiasis in adult patients, a single-centre, parallel group, open-label, randomized non-inferiority trial was carried out. One-hundred and 26 participants who had symptomatic Giardia mono-infection took part in the study. Direct wet mount and/or Ritchie concentration techniques and physical examinations were conducted at the time of enrolment and at the follow-up visit. The primary outcome measure was parasitological cure, performed at 3, 5, 10 days post-treatment. Negative faecal specimens for Giardia were ensured by the same parasitological techniques. At follow up (day 10) the parasitological cure rate for the per protocol populations was 88.7% (55/62) for MBZ and 91.8% (56/61) for SNZ. For the intention to treat populations the cure rate at the end of treatment was 85.9% (55/64) for MBZ and 90.3% (56/62) for SNZ. Both analyzes showed there was not significant statistical difference between MBZ and SNZ treatment efficacy. Both drugs were well tolerated, only mild, transient and self-limited side effects were reported and did not require discontinuation of treatment. A 3-day course of mebendazole seems to be as efficacious and safe for treatment of giardiasis as a single dose of secnidazole in adults. 1. Introduction Giardia lamblia, the causative agent of giardiasis, is one of the commonest intestinal parasitic protozoan infections diagnosed world-wide. The spectrum of this infection may range from asymptomatic shedding of giardial cysts to symptomatic giardiasis, being responsible for abdominal cramps, nausea, acute or chronic diarrhoea, with malabsorption, and failure of children to thrive [1]. Five nitroimidazole compounds are considered to be the first-line regular therapy for this infectious disease. However, whilst their therapeutic benefits are generally accepted, treatment failures are often reported [2, 3], and that is why a variety of new approaches to the treatment of giardiasis have been entering into clinical practice [4]. Evidence from uncontrolled case series and clinical trials in paediatric patients suggests that mebendazole (MBZ) might have a role in the treatment in this parasitosis and that its therapeutic effect is achieved without an accompanying increase of side effects [5–8]. Despite the number of articles published concerning the use of this drug in paediatric patients with giardiasis, the information about the use of MBZ in adult giardiasis is scarce. The aim of the study was to determine whether MBZ is as efficacious and safe as
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