I like the idea of modifying existing drugs. It's a much more elegant solution than what CGL went for.
In regard to the vectors I have to say: Injection should always be an option, no matter the other vector, because your bloodstream is always the place where you want the drug to end up.
Contact usually needs an added agent that will allow the drug to permeate into your body. DMSO is a reliable solution for that, so in most cases you don't really need to complicate your drug (you might still want to if DMSO is to expensive for your desired product.)
Likewise, ingestion always works too - albeit slower both for feeling the effects but also for a longer duration.
Inhalation can either be achieved through heating/burning (smoking, obviously) or through bringing it into solution and then using a vaporizer.
The hierarchy of effects could be like this:
Injection: Immediate effect, +1 to positive and negative effects, half duration, +2 to Addiction Threshold
Smoking/Inhaling: Immediate effect, normal duration, +1 to Addiction Threshold
Contact: One to two rounds effect, double duration
Ingestion: Three rounds or more until effect, quadruple duration, -1 to positive and negative effects, -1 to Addiction Threshold
(If I remember my few lessons on biochemistry correctly
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