Cis atracurium is preferred over atracurium due to advantage of:

Cis atracurium is a stereoisomer of Atracurium and is responsible for most pharmacological propeies. Unlike atracurium, cisatracurium does not produce a consistent dose dependent increase in plasma histamine. Another advantage of cis atracurium is that it produce less laudanosine. Atracurium triggers dose dependent histamine release that becomes significant at dose above 5mg/kg. The release is dose dependent such that with increasing dose administered at the same rate there is greater propensity for eliciting histamine release resulting in bronchospasm and hemodynamic instability.