Dopamine injections into the cactus as it grows would increase it potency because dopamine is a biological precursor to the active component. Dopamine is a controlled substance as well however, but it is found in banana peel and is water soluble. In fact dopamine has been measured in banana peel at a value of up to 100mg dopamine/g of peel.

Save your banana peels in a big baggie in the freezer and when you got a huge bag full of frozen peel grind them up and simmer them in a huge pot with distilled water.
decoct them for a long time filter off the ground peel pulp and boil off most of the water. allow to cool and water/inject you cactus with the aqueous banana extract.

Humidity plays a role as well cactuses grown in humid environments are gennerally less potent than dry environments. Look for darker cactuses. Dark green are more potent than lighter colored cactuses.