If you enter a "not guilty" plea, then the matter will be set for a trial. If it goes to trial it could drag out for months and if you lose, you should know the fees are about twice the amount as non-trial cases, and the chances of you winning, given my experience, are pretty slim.
That said, if you ABSOLUTELY DID NOT consume, then pleading not guilty is well within your rights. If you just want to plead not guilty to make them prove their case, then please, don't waste everybody's time... it's not the crime of the century... really. Pay the fine or whatever... and move on. It's just not worth the hassle.
10 years as a probation officer.