The science of it is easy:
The more blood that pumps to your cock, the bigger it will be. Thus, whatever you're doing / taking should be to increase blood-flow.
Once the Corpus cavernosa is always FULL with blood, it will expand on its own (thus giving you a larger penis).
So my opinion is, any drug that will increase blood-flow to the penis, will work and shouldn't have any side effects. Others that work on a different theory would have to be judged on a per case basis
Or you can just do normal things like have a healthier diet and more exercise. Increased frequency of sexual relations also helps.'Jilqing' or whatever its called probably also helps increase your blood-flow.
This isn't rocket science, it's simple anatomy
