The future method of choice for clandestine drug manufacture.
NB: I say future, but this method is perfectly viable now. It has not, though, caught on.

A nuumber of recreational drugs act on receptors normally stimulated by endogenous proteins. Heroin acts on the mu-opioid receptor. Δ-9-tetrahydrocannabinol, THC, acts on the THC receptor. The endogenous proteins for these receptors are β-endorphin and anandamide, respectively.

These proteins can produce a recreational "high".
These proteins are not illegal.
Therefore, there is a market for these proteins.

The DNA snippets can easily be obtained, as the sequences are published.
The restriction procedure and plasmid transfer process I will leave as an exercise for the reader.
Once introduced into a suitable bacteria, the protein is manufactured by letting the bacteria reproduce in a nutrient substrate.
Production can be large-scale and extremely cheap.

It's as easy as baking a cake.
A really small cake, at least.