  This paper presents a method for shaping the transmit pulse of a molecular
signal such that the diffusion channel's response is a sharp pulse. The impulse
response of a diffusion channel is typically characterised as having an
infinitely long transient response. This can cause severe
inter-symbol-interference, and reduce the achievable reliable bit rate. We
achieve the desired chemical channel response by poisoning the channel with a
secondary compound, such that it chemically cancels aspects of the primary
information signal. We use two independent methods to show that the chemical
concentration of the \emph{information signal} should be $\propto \delta(t)$
and that of the \emph{poison signal} should be $\propto t^{-3/2}$.
