Narcotic alkaloid C17H19NO3 derived from
opium and prescribed only to alleviate severe
pain. Its use produces serious side effects,
including nausea, constipation, tolerance,
and addiction, but it is highly valued for
the relief of the terminally ill. It is a
controlled substance in Britain.

morphogen

In medicine, one of a class of substances
believed to be present in the growing embryo,
controlling its growth pattern. It is thought
that variations in the concentration of
morphogens in different parts of the embryo
cause them to grow at different rates.