Polymorphine

Polymorphine is the hallmark and exotic drug of the Callidus Assassins. when combined with the extensive Callidus training, allows the assassin to change her flesh to adopt the appearance of almost any human or even a humanoid alien. It can be used to adopt a specific appearance so long as the user has seen and studied the target, or that of a more general disguise when detailed change is not necessary.

A single dose of polymorphyne lasts for two standard hours, but its effects can be mitigated at will by the user. Those untrained by the Callidus temple can only assume shapes that approximate their own, as they lack the surgical implants necessary for more radical transformations. Even then, it takes a strong will to control the drug's effects, risking transformation into an abhorrent amalgam of their current and desired forms for the next half-hour.

The transformation itself requires a bare minimum of five minutes to complete. In that time, the user is quite terrible to behold as their flesh molds to their desired form.

Source

 * Dark Heresy (2nd Edition) -- Enemies from Beyond (pg 41).