Flexibility of metabolites and enzymes is investigated (i) on the level of the individual molecule, (ii) on the pathway level and (iii) combined effects on the systems and network level. Tools and results from our current research are summarized including data from our metabolite enzyme database. Including our latest census we find frequently used metabolites stimulate evolutionary flexibility in specific enzyme superfamilies. Furthermore, simultaneous changes of reactions and metabolites are observed in these flexible enzyme superfamilies. Both effects provide a strong source for resistance in parasites and pathogens. Specific adaptations scenarios and some counter strategies are discussed.