Or/and try propolis:

Samet N, Laurent C, Susarla SM, Samet-Rubinsteen N.
The effect of bee propolis on recurrent aphthous stomatitis: a pilot
study.
Clin Oral Investig. 2007 Jun;11(2):143-7. Epub 2007 Feb 7.
PMID: 17285269 [PubMed - in process]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=17285269>

   "Recurrent aphthous stomatitis (RAS) is a common, painful,
   and ulcerative disorder of the oral cavity of unknown
   etiology. No cure exists and medications aim to reduce pain
   associated with ulcers through topical applications or reduce
   outbreak frequency with systemic medications, many having
   serious side effects. The purpose of this pilot study was to
   evaluate the potential of a product to reduce the number of
   outbreaks of RAS ulcers. Propolis is a bee product used in
   some cultures as treatment for mouth ulcers. In this
   randomized, double-blind, placebo-controlled study, patients
   were assigned to take 500 mg of propolis or a placebo capsule
   daily. Subjects reported a baseline ulcer frequency and were
   contacted biweekly to record recurrences. Data were analyzed
   to determine if subjects had a decrease of 50% in outbreak
   frequency. The data indicated a statistically significant
   reduction of outbreaks in the propolis group (Fisher's exact
   test, one sided, p = 0.04). Patients in the propolis group
   also self-reported a significant improvement in their quality
   of life (p = 0.03). This study has shown propolis to be
   effective in decreasing the number of recurrences and improve
   the quality of life in patients who suffer from RAS. Propolis
   should be evaluated further in a larger sample clinical
   trial."