Tolerability of IM penicillin G benzathine diluted or not with local anesthetics, or different gauge needles for syphilis treatment: a randomized clinical trial

Background Penicillin G Benzathine (PGB) is the cornerstone of syphilis treatment. However, its intramuscular (IM) administration is associated with pain at the site of injection. The dilution of PGB with local anesthetics is recommended in some guidelines, but the evidence that supports it, particularly in adults and in HIV infection, is scarce. Preliminary clinical experience also suggests that the IM administration of PGB through increased needle gauges might improve its tolerability. The aim of the study to identify less painful ways of administering IM PGB in the treatment of syphilis in adults. Methods Multicenter, randomized, double-blinded clinical trial in patients diagnosed with primary syphilis that required a single IM injection of PGB 2400,00 IU. Patients were randomized to receive PGB diluted with 0.5 mL mepivacaine 1% (MV) or PGB alone, and both groups either with a long 19G or short 21G IM needle. The primary objective was the effect on local pain immediately after the administration through a visual scale questionnaire on pain (0 to 10). Results One hundred eight patients were included, 27 in each group. Ninety-four (94.4%) were male, and 41.7% were also HIV-infected. Mean age 36.6 years (SD 11). Significant differences in immediate pain intensity were observed when comparing the long 19G group with anesthesia (mean pain intensity, [MPI] 2.92 [CI 95% 1.08-4.07]) vs long 19G without anesthesia (MPI 5.56 [CI 95% 4.39-6.73), p p = 0.015). No significant differences in immediate pain were observed between 19G and 21G in the presence or absence of anesthesia (p = 1.0 in both cases). No differences were found between study arms after 6 and 24 h. Conclusions The IM administration of 1% mepivacaine-diluted PGB induces significantly less immediate local pain as compared to PGB alone. The needle gauge did not have any effect on the pain. Based on these results, we suggest anesthetic-diluted IM PGB as the standard treatment for primary syphilis. Trial registration EudraCT 2014-003969-24 (Date of registration 18/09/2014).