Why didn't Walter White ever consume his own product in Breaking Bad? I mean he did once consume cannabis, is this because of work ethics or any alternate views?
The series was not really so much about drug use, and if Walter had been a tweaker then it presumably would have had to have been more so. Also, it would have overshadowed some of the ambiguities and nuances of his character; getting high would in fact have been a good explanation for some of his behaviour but having such an easy explanation would have made it less thought provoking.
I don't, as other people have said, think it has anything to do with Walter's personality. He could just as easily have been using without changing the plot (I think what is in keeping with his personality would be that he would conceal his use from others), but again, the focus of the story would then be a drug user and the series was not about drug users; making it more so would have diluted other elements.
There are really surprisingly few scenes of users using considering the length of the series; much of that is just Jessie getting stoned on weed -- and even more than that, just Jesse sitting smoking a cigarette.
Most of the criminal characters aren't users and show some disdain for them. In a way, drugs are totally tangential to the plot; they just happen to be a source of income for organized crime, which is the focus of Breaking Bad, as per Vince Gilligan's famous quote about how he wanted to take Mr. Chips and transform him into Scarface.
Of course the meth thing is very sexy; although the plot could as easily revolved around smuggling guns or diamonds, having someone actually making drugs on TV was a bit more original and interesting. But the show was still not about drug use, it was about criminality and violence.
He's a Science teacher. He understands the effects of Meth and knows how addictive it is. Besides, he was in it to make money. Never combine business with pleasure if you want to be successful. Just as Frank Lopez warns Tony Montana in Scarface, "Don't get high on your own supply".
Walter never wanted to be addicted to the stuff he was making. He was already sick because of cancer and doing drugs could worsen his health and as Johnny explains in his answer that being Chemistry wiz he knew the ill effects. He wanted to be alive and awake all the time so that he could take clever decisions and complete his job no matter how difficult it was, be it packing & transferring meth, or killing someone. It would have been difficult for him to manage everything he was doing and lying to people would have been difficult for him.
He also had to return to his home every night and sit with his family plus his brother in law was in the DEA and maybe he could sniff meth because that was everything he looked for. Maybe he also felt mature to do stuff like this and he also had kids whom he never wanted to influence.
Walter is "in the business of building empires." There is a clear cultural separation where if you want to be in the business and earn the respect of people like Gus, you need to be firmly above the product.
I think talking about the effects of meth, etc. dilutes this point. Walter isn't tempted at all; that would violate his vision of himself, and his goals. Yes, Jesse, in particular his relationship with Walter, is very complicated because Jesse uses but it otherwise nearly as talented as Walter.
I agree with the above stated answers that Walt didn't want to become addicted, needed to stay focused, etc. But I feel the biggest reason was his pride. I think he felt he was always better than those crank addicted junkies. This is evidenced on how he talked down to Jessie. Walt was a scientist "for chrissake!" and he wanted everyone to know this was business, he wasn't a junkie.
But it was his pride too that got the better of him, turning him darker in the guise of him helping his family. He was actually becoming the scum he hated, and Jessie was becoming the concienscious person Walt pretended Walt was all along.
True drug dealers or any business man know the power and influence of consuming one's own products, and good drug dealers and manufacturers "never get high over their own supply."
Walter knew the influence drugs could have and as a chemist and synthesist, he knew it was too dangerous to consume even for enjoyment. Walter wanted to be someone big, famous, respected, and rich and no one that popular was known for consuming meth.