When I splice an indica branch onto a sativa, that's genetically modifying the plants. When we cross breed, force hermie, and then back breed for stabilization, that's genetically modifying our plants. So, I assume that you only grow/consume outdoor grown, pure strain indicas and sativas? Otherwise, you ARE consuming genetically modified plants. Whether it's the splicing of a single gene or a selective splice/breed/back breed, it's all genetic modification.
Multivitamins are a supplement, and should only be taken if necessary. Eating a balanced, whole-food based diet, with an emphasis on dark greens, legumes, nuts, and whole grains will give you all of the vitamins and minerals that you normally need. I take creatine -- because I don't eat red meat more than 1 or 2 times a month. I don't need to supplement anything else though For me, it's actually the most difficult to keep up with my caloric intake needs some times (around 6500 daily, during the work season.) My blood tests for DOT and for routine testing always shows I am a specimen of model health. I smoke cigarettes, and between 4 -8 grams every single day.
The concept of everyone taking a daily is actually quite dangerous. Here is the first link that came up in a google search, referencing multiple studies.
Are multis good or bad?
With the Harvard Study. Harvard is a second rate University, but still is a world class research facility. They've done a great deal of groundbreaking research on Molta, and the physiological effects on the human body, including it's possible negative affects, and it's medicinal properties. My favorite study from Harvard kinda' contradicts your "pot is bad" study. Harvard Journal of Medicine, circa 2007, Marijuana and Lung Cancer. Long Stem, Short Stem -- both HALTED and in some cases REVERSED when treated with Marijuana.
Here's a story about the study.