the five precepts and marijuana - does marijuana count as an intoxicant? The fifth of the precepts is &quot;I undertake the training to refrain from intoxicants.&quot;
does smoking marijuana count as an intoxicant ?
Yes. Marijuana is an intoxicating substance.
From AccessToInsight.org's The Five Precepts: pañca-sila:


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 &quot;A Discipline of Sobriety&quot; 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 cocktail
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, &quot;the way to the Deathless,&quot; 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.