The tablet form is not made for IV conversion. Even the IV form is not used often, it a a high-risk procedure with the potential for severe tissue damage including gangrene. The preferred route of administration for Promethazine injections is deep intramuscular (IM). If you are looking for more of a feeling, try to crush the tablet and place under the tongue to dissolve.
Levomepromazine can be IV'd but it says to dilute with saline first. Here is a link on it :