The article presents an analysis of paronymic Russian prefixal verbs converting into synonyms in colloquial speech. Paronyms and synonyms are at first sight distant and opposite lexical concepts. The former underline semantic differences between words whereas the latter focus on their similarities. However, common-root derivatives feature both, being different in affixes while sharing the root. The same lexical units can be regarded either as paronyms or as synonyms depending on particular theoretical models and on what is in focus – their contrast or their resemblance. Lexical and grammatical norms preserve the distinction between close meanings of paronyms, while dynamic language processes erase these differences. We examine prefixal paronyms drifting to synonyms in colloquial speech and suggest reasons for such phenomenon. Synonymy becomes possible when prefixes partly coincide in meaning (one meaning included into the other, or two meanings overlapping due to the shift of the action landmark). Productivity of a derivational model, phonetic resemblance of prefixes, and tendencies towards analogy and economy in colloquial speech are also to be taken into account.