taking vows (the Five Precepts) and magic mushrooms

can I use them one last time

My own experience says "no".

For me a typical cycle is like this:

• Try it
• Like it
• Want it again

and then:

• Have it many times
• Eventually become sick of it -- at this point, I might think I ought to quit, and that I probably can but don't want to quit quite yet -- and "finally" decide I will quit or try to quit.

Then, having decided to quit:

• I'm quitting now so it's OK, I'll just try it once more
• As you said in the OP, "have that experience one last time":
  • experience it again
  • and then experience its ending/cessation again
• Want it again!

The problem now is that having decided it's OK to try it "one last time" the same logic now still holds -- it's OK to try it one last time. And so this becomes the new cycle, once becomes twice, and the circumstances (including sobriety) in which quitting seemed plausible becomes more remote. Meanwhile the harm accumulates.

Theoretically perhaps someone less addicted or less "addictive" than I am might be able to use something "one last time", but experience tells me I'm unable, and that instead "once" becomes "again" and so on. For me, the only way to stop is to stop, never, not once -- to have already stopped.

Here's from Piya Tan's analysis of SN 56.11:

The Dhamma,cakka Pavattana Sutta fully and clearly explains the functions of each of the 4 noble truths, thus:

1. The 1st noble truth, that is, suffering is to be understood.
2. The 2nd noble truth, that is, the arising of suffering is to be abandoned.
3. The 3rd noble truth, that is, the ending of suffering (nirvana) is to be realized.
4. The 4th noble truth, that is, the way to the ending of suffering is to be cultivated.

The Sutta is also unique in presenting the 3 phases (ti,parivaṭṭa) of each of the truths, namely:

1. the knowledge (ie, understanding) of the truth, sacca,ñāṇa
2. the knowledge of the task to be done regarding each truth, and kicca,ñāṇa
3. the knowledge of the accomplishment of these tasks. kata,ñāṇa

The commentarial version of these phases is called “the 3 good truths” (saddhamma):

1. the true Dharma as theory pariyatti.saddhamma (textual aspect),
2. the true Dharma as practice paṭipatti.saddhamma (moral virtue and meditation), and
3. the true Dharma as realization paṭivedha.saddhamma (sainthood and freedom).

When these 3 phases are applied to each of the 4 truths, they total as the 12 modes or aspects (dvādas’-ākāra).

Note what it says about "phases" and "knowledge of accomplishment".

Quoting from SN 56.11 itself:

Vision arose, insight arose, discernment arose, knowledge arose, illumination arose within me with regard to things never heard before: 'This is the noble truth of the origination of stress'... 'This noble truth of the origination of stress is to be abandoned' ... 'This noble truth of the origination of stress has been abandoned.'

"Vision arose, insight arose, discernment arose, knowledge arose, illumination arose within me with regard to things never heard before: 'This is the noble truth of the cessation of stress'... 'This noble truth of the cessation of stress is to be directly experienced'... 'This noble truth of the cessation of stress has been directly experienced.

The "has been" there is an example of the grammatical perfect tense -- i.e. to have already quit.

On the plus side I can tell you that I very much don't regret -- I feel no remorse about -- the decision to stop completely.

It reminds me of this paragraph ...

For the Ariyan Dhamma, the distinction between the two types of conduct, moral and immoral, is sharp and clear, and this distinction persists all the way through to the consummation of the path: "Bodily conduct is twofold, I say, to be cultivated and not to be cultivated, and such conduct is either the one or the other" (MN 114). The conduct of the ideal Buddhist sage, the arahant, necessarily embodies the highest standards of moral rectitude both in the spirit and in the letter, and for him conformity to the letter is spontaneous and natural. The Buddha says that the liberated one lives restrained by the rules of the Vinaya, seeing danger in the slightest faults. He cannot intentionally commit any breach of the moral precepts, nor would he ever pursue any course of action motivated by desire, hatred, delusion, or fear.

... i.e. "seeing danger in the slightest faults".

I see danger!

And in fact I have quit completely and no longer have the "habit", and more-or-less "uprooted" the "conditions" in which even temptation might occur -- the thought (of "using them once last time") almost never ever occurs -- which I find relatively very pleasant (e.g. compared with "danger").

I'm not claiming to be an Arahant but I figure it's important to be careful about what behaviour I allow myself -- especially habitual behaviour.

Q: does cannabis and magic mushrooms count as intoxicants?

If it can make you heedless or careless, and fall away from observing the first four of the five precepts, then yes, it is considered an intoxicant under the fifth precept.

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

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.

Please also see this answer.

Q: does masturbation count as sexual misconduct?

For lay persons, sexual or sensual misconduct (defined in MN 41) is more about adultery, infidelity, and a sensual relationship outside of a committed exclusive relationship (or marriage) with a free consenting adult (of legal age), rather than a specific sexual practice or fetish.

The purpose of this precept is to remove potential animosity with others and also to remove the remorse of hurting others.

Please also see this answer.

Yes all drugs that affect the mind and causes heedlessness are intoxicants in both the most conventional sense as well as in Buddhist morality. The issue here is to what point is heedlessness. If you do some study on this subject in suttas you may find out when you are high on cannabis your mind is not heedless. Mine was not when I used to smoke weed regularly but it impeded my meditation practice so i gave it up...eventually. Essentially heedlessness means unable to control your mind and the defilements which in turn cause you to create negative causes. Either way all drugs are a waste of time and are obstacles to meditation.

This is coming from someone who has taken loads of different drugs, used to bee a weed head and am still a boozer.

Masturbation is not sexual misconduct in of itself except for a monk as monks (are supposed to) refrain from all sexual activity. The 2 issues here is your sperm retained is a powerful tool you can use to control your mind and meditation that you should not waste, and more importantly coveting another person is bad for your mind in helping you curb and control desire. What is sexual immorality is coveting a non sexual object, so a child, a wife, or someone girlfriend or of course your own mother or sister. Gender swap roles if you are a female or h*m*sexual of course. Covetousness is never good either way and I would suggest to only use certain people who you don't covet in your fantasies for only arousal to release or even p**n just to arouse you for a release rather than watching it to sustain desire or entertainment.

Further more if at present you are unable to take solemn vows of refraining from intoxication and/or sexual misconduct, you can still take solemn vows of refraining from killing, lying, stealing and when you are truly able to keep to the vows of refraining from intoxication and sexual misconduct, then add them to your already established vows. Start small if you need to, essentially it is all Sila, morality and a gradual practice of turning your mind from non virtue to virtue.