Acquisition and enrichment of lexical resources is an important research area for the computational linguistics. We propose a method for inducing a lexicon of synonyms and for its weighting in order to establish its reliability. The method is based on the analysis of syntactic structure of complex terms. We apply and evaluate the approach on three biomedical terminologies (MeSH, Snomed Int, Snomed CT). Between 7.7 and 33.6% of the induced synonyms are ambiguous and cooccur with other semantic relations. A virtual reference allows to validate 9 to 14% of the induced synonyms.