Cayno group bind very actively and form strong bond with the adeherent substance or matrix or base or a competing molecule. If any thing bind with high affinity to any bio-material it will show strong toxic effect.  

If it binds strongly then it should also exert strong biological effect? It means if we add cyano group to any bioactive molecule then would it increase its activity or only add to its toxicity?

Cyano gp has different values of electronic effect (as electron withdrawing gp), resonance effect (as a substituent in aromatic systems), lipophilic effect and steric effect. The size and the volume is much higher than fluoro substituent. An additional point also, is about it's metabolism: cyano gp will hydrolize to amide and then to carboxylic gp.  Finally it's a carbon triply bonded with a nitrogen, thus represents an extension of the structure.