Benzydamine cream for the treatment of post-herpetic neuralgia: minimum duration of treatment periods in a cross-over trial.

In a double-blind multiple-dose cross-over study benzydamine 3% cream was compared with placebo for the treatment of post-herpetic neuralgia. Pain relief, pain intensity, sleep, escape analgesic consumption and side effects were assessed by diary methods for the 2 week treatment periods, with 1 week run-in and 1 week wash-out. There were no significant differences between the 2 treatments. The implications of the results for other antiprostaglandin remedies recommended for treatment of post-herpetic neuralgia are discussed. An important observation with methodological significance for similar studies of chronic conditions was that short treatment periods may produce false positive results. Patients' expectations are high, and if the first study treatment is ineffective, initial significant benefit may be noted when crossing over to the next treatment; this may not last longer than 1 week. Cross-over studies in which neither treatment is effective may, therefore, produce erroneous results if treatment periods are shorter than 2 weeks.