the five precepts and marijuana - does marijuana count as an intoxicant?

Yes, it is.

SurāMerayamajjaPamādatthānā VeramaniSikkhāpadam Samādiyāmi

I choose (undertake) to manage the basis of restrain-oneself-practice instead of the management of the craziness's basis and stupidness's basis, which causing unwholesome mindfulness ( forgotten the meditation object and losing the control of mind and actions which might cause breaking other precept easily).

When you are smoking marijuana, it's hard to thinking of the meditation object or any practice lesson.

If one still addict on some intoxicants, he can re-making attentions by reciting 5 precepts everyday after hanging because it reminds him what he should do. This is better than worrying and faking himself whether "I feel better when I forget 5 precepts, and I'm not ready to keep 5 precepts, so I will undertake/recite it next time when I'm ready." Because no one is ready at first. Everyone has been fail before. So don't care of "I am still smoking marijuana", just care of "re-recite 5 precepts everyday". One day after that in many years, this one is going to be better.

Yes, if used recreationally it is obviously against the precept. It is distracting, makes one less serious, careless and indulgent.

There are also a lot of drawbacks as in withdrawal when quitting.

Nicotine and Caffeine are also intoxicants.

Yes. Marijuana is an intoxicating substance.

From AccessToInsight.org's The Five Precepts: pañca-sila:

5. Suramerayamajja pamadatthana veramani sikkhapadam samadiyami
   I undertake the precept to refrain from intoxicating drinks and drugs which lead to carelessness.

It comes from the suttas e.g. AN 8.39.

The reason for abstaining from alcohol, drugs and any other intoxicating substances is explained below. It's because intoxicating substances cause heedlessness and moral recklessness, and results in one breaking the other four precepts, because a person's best judgment is impaired under the influence of intoxicating substances.

However, if intoxicating substances are consumed in a strictly controlled way as a medicine, administered by a medical professional, then it is ok. For e.g. I have heard that psilocybin has been used for treating severe depression.

From the essay "A Discipline of Sobriety" by Ven. Bodhi (the highlighted statements apply to both alcohol and drugs):

For his lay followers the Buddha has prescribed five precepts as the minimal moral observance: abstinence from killing, stealing, sexual misconduct, false speech, and the use of intoxicants. He did not lay down these precepts arbitrarily or out of compliance with ancient customs, but because he understood, with his omniscient knowledge, which lines of conduct lead to our welfare and happiness and which lead to harm and suffering. The fifth precept, it should be stressed, is not a pledge merely to abstain from intoxication or from excessive consumption of liquor. It calls for nothing short of total abstinence. By this rule the Buddha shows that he has understood well the subtle, pernicious nature of addiction. Alcoholism rarely claims its victims in a sudden swoop. Usually it sets in gradually, beginning perhaps with the social icebreaker, the drink among friends, or the c**ktail after a hard day's work. But it does not stop there: slowly it sinks its talons into its victims' hearts until they are reduced to its helpless prey.

To dispel any doubt about his reasons for prescribing this precept, the Buddha has written the explanation into the rule itself: one is to refrain from the use of intoxicating drinks and drugs because they are the cause of heedlessness (pamada). Heedlessness means moral recklessness, disregard for the bounds between right and wrong. It is the loss of heedfulness (appamada), moral scrupulousness based on a keen perception of the dangers in unwholesome states. Heedfulness is the keynote of the Buddhist path, "the way to the Deathless," running through all three stages of the path: morality, concentration, and wisdom. To indulge in intoxicating drinks is to risk falling away from each stage. The use of alcohol blunts the sense of shame and moral dread and thus leads almost inevitably to a breach of the other precepts. One addicted to liquor will have little hesitation to lie or steal, will lose all sense of sexual decency, and may easily be provoked even to murder. Hard statistics clearly confirm the close connection between the use of alcohol and violent crime, not to speak of traffic accidents, occupational hazards, and disharmony within the home. Alcoholism is indeed a most costly burden on the whole society.

When the use of intoxicants eats away at even the most basic moral scruples, little need be said about its corrosive influence on the two higher stages of the path. A mind besotted by drink will lack the alertness required for meditative training and certainly won't be able to make the fine distinctions between good and bad mental qualities needed to develop wisdom. The Buddhist path in its entirety is a discipline of sobriety, a discipline which demands the courage and honesty to take a long, hard, utterly sober look at the sobering truths about existence. Such courage and honesty will hardly be possible for one who must escape from truth into the glittering but fragile fantasyland opened up by drink and drugs.

It may well be that a mature, reasonably well-adjusted person can enjoy a few drinks with friends without turning into a drunkard or a murderous fiend. But there is another factor to consider: namely, that this life is not the only life we lead. Our stream of consciousness does not terminate with death but continues on in other forms, and the form it takes is determined by our habits, propensities, and actions in this present life. The possibilities of rebirth are boundless, yet the road to the lower realms is wide and smooth, the road upward steep and narrow. If we were ordered to walk along a narrow ledge overlooking a sharp precipice, we certainly would not want to put ourselves at risk by first enjoying a few drinks. We would be too keenly aware that nothing less than our life is at stake. If we only had eyes to see, we would realize that this is a perfect metaphor for the human condition, as the Buddha himself, the One with Vision, confirms (see SN 56:42). As human beings we walk along a narrow ledge, and if our moral sense is dulled we can easily topple over the edge, down to the plane of misery, from which it is extremely difficult to re-emerge.

But it is not for our own sakes alone, nor even for the wider benefit of our family and friends, that we should heed the Buddha's injunction to abstain from intoxicants. To do so is also part of our personal responsibility for preserving the Buddha's Sasana. The Teaching can survive only as long as its followers uphold it, and in the present day one of the most insidious corruptions eating away at the entrails of Buddhism is the extensive spread of the drinking habit among those same followers. If we truly want the Dhamma to endure long, to keep the path to deliverance open for all the world, then we must remain heedful. If the current trend continues and more and more Buddhists succumb to the lure of intoxicating drinks, we can be sure that the Teaching will perish in all but name. At this very moment of history when its message has become most urgent, the sacred Dhamma of the Buddha will be irreparably lost, drowned out by the clinking of glasses and our rounds of merry toasts.