The following are 4 regulative principles everyone should follow regardless they are in spiritual life or not. Every scripture mentions it again and again. Srila Prabhupada emphasised on it prior to accepting a discipline, and so do any Rasika Vaishnavas.
These 4 principles are based on 4 pillars of dharma - austerity, cleanliness, truthfulness and compassion. This is a must-follow for a sincere spiritual seeker.
These regulative principles are required to be followed to be normal human first, the bhakti comes later, hence these 4 principles have been stressed by Rasika Vaishnavas, Vedic scriptures and saints again and again.
No Intoxication
The scriptures absolutely bar one from drinking alcohol and consuming intoxicants (This principle is based on the pillar of austerity).
The eventual end-point of human life is to constantly meditate (Bhagavad Gita 18.66) and serve the Divine Couple Radha and Krishna in nitya-lilas of Lord in an eternal individual relationship, and getting influenced by external intoxicants bars the devotee or jiva (human being) from meditating on Lordships.
Anyone doesn't need to mention the harmful effects of intoxicants, and frankly, everyone is aware of it.
Someone may ask, but why did Krishna make this these stuffs in the creation that are barred in scriptures? Rasika Vaishnavas mention that Krishna didn't make it, in fact, we wanted to enjoy and be independent and Krishna kindly provided these facilities in the form of the material world.
It is now our choice to keep getting trapped in all this prapanch (non-sense) and illusion or get out of it by desperately seeking our real state of sacidananda (eternal, full of knowledge, and full of bliss).
The book 'Demystifying The Mystical Vrindavan' is the essence and crux of this entire website.
If you are considering to dive into the final goal of our Vedic Scriptures and spiritual life, and if Radharani inspires you, do get a copy of the book .
If you don't believe this, just simply chant Hare Krishna Hare Krishna | Krishna Krishna Hare Rama | Hare Rama Rama Rama | Hare Hare and read these books for spiritual advancement (if inspired from within). All your questions on these will be fully answered in due course of time.
Eventually, chanting Hare Krishna Mahamantra is the endpoint of it all, whether you accept it today or 50 lifetimes later!
Though I don't make authoritative statements usually unless inspired from within by Srimati Radharani, but I am saying this fully convinced by listening to the vani (words) and experiencing kripa (mercy) of Rasika Vaishnavas and Sri Vrindavan Dham first hand. Rest however Srimati Radharani inspires you.
No Meat Eating
No flesh-eating or killing animals (This principle is based on the pillar of compassion)
A spiritual seeker (sadhaka) is full of compassion for everyone, as they see their prana-aradhya Divine Couple in everyone. They can't hurt and kill anybody for own enjoyment in the form of food.
Though, even if you look at it with all that scientific logic and references, there are thousands of case studies that prove that animal food is not meant for our stomach as human intestines can't really absorb and digest the animal flesh in best means. If you want references, comment below and I will share a few authentic references with you (Though it's just a matter of false pride and might be irrelevant but I had really gone in-depth of these subjects way back in 2014 during my initial days in Krishna Consciousness).
No Gambling
No gambling or playing with fate (This principle is based on the pillar of truthfulness)
The concept of sincere seekers (as taught by Rasika Vaishnavas and Vedic Scriptures) is to surrender fully to Lord and depend on Him by doing honest, sincere effort in whatever work (permitted by scriptures) they are doing in order to maintain body and other social responsibilities, and eventually offering it to Him.
The prahdanta (importance) is of chintan (meditation) onto the Divine Couple Radha and Krishna and chanting the holy names Hare Krishna Hare Krishna | Krishna Krishna Hare Hare | Hare Rama Hare Rama | Rama Rama Hare Hare even while performing our allotted seva of whatever honest occupation we are directed with by the Lord. Sriji arranges everything for sincere devotees.
Gambling and all short term illegal stuff drift you away from bhajan (spiritual practice) and do not make you depend on Lordships, hence is absolutely forbidden for any human being.
No Illicit Sex
No sexual relationship with anybody except a religiously-wedded wife and that too for procreation of Krishna Conscious children.
Procreation and sex are meant for procreation of a Krishna Consciousness child. In Bhagavad Gita (7.11), Krishna says that he is the sex life that is not contrary to religious principles.
Though sex only for the procreation of child is the ideal standard of Vedic Scriptures and advocated by Rasika Vaishnavas, I have also heard from them that married couple can support each other and help each other do more bhajan and chanting, and by engaging in sexual activity less gradually if they are unable to control their urge.
They also mention that it is the conditioning of many lifetimes that one is fighting with, and it can be easily overcome by chanting the holy names and fully surrendering to Krisna in the form of a bonafide Rasika Guru, and honestly following their instructions. Grishasta ashram (marriage life) is not meant to enjoy sexual life but rather a means to support each other to eventually advance in bhajan by chanting intensely and helping each other remember lilas of Lordships, reading scriptures, serving Vaishnavas and rendering sincere loving devotional service.
Concluding Note
All the 4 regulative principles are thought of a big thing by general masses today. This is the effect of Kali Yuga and is not easy to just listen to these instructions unless one begins to chant holy names of Lord in form of Hare Krishna Mahamantra sincerely, comes to Vrindavan regularly, studies books by Rasika Vaishnavas, associates with Rasika devotees of Lord and has prasadam (food offered to Lord) that is made with love.
Also, scriptures and Rasika Vaishnavas mention not to eat onion, garlic, tea, coffee (basically caffeinated food) as these are tamasic (mode of ignorance) food and enhance the sexual desire and stimulate the brain into thinking negative. There are proper Vedic references on how onion and garlic are Rahu and Ketu, who are of asuric (demonic) nature.
People may again mention that garlic has medicinal properties and all that reasoning but the very fact is that there are two modes - good (sura) and bad (asura), and eventually you have to choose - you wish to take the way prescribed by Krishna and His devotees, or listen to your imperfect mind (That's what the scriptures say, precisely Bhagavad Gita 6.6).
Again, I can answer in all those scientific details but who cares. Who has to follow will follow and who doesn't feel it be true, feel free to go the long way. You don't really need to read this. Eventually, Maya Devi (Krishna's illusionary force) is very strong and history has been a proof that the best of tricksters have been tricked by the father of all world-class tricksters, Sri Krishna.
Whenever you feel you are about to lose, don't forget the bliss Krishna can provide you!
This spiritual path is for the fearless and determined people. Don't lose hope ever. Anybody who takes a step towards Krishna, Krishna approaches them by coming closer 1000 steps. He is most merciful (karuna mayi) and once somebody takes His shelter, they never have to worry about anything. Leave it up to Him and He will carry you personally, though He does that for everybody BUT once you surrender your heart and ownself to Him, you can experience it first hand and appreciate His karuna (mercy). Read this confidential secret for advancing in bhakti very fast! This would be a great cheat code to grow spiritually really fast (which I have heard from Rasika Vaishnavas in Vrindavan).
Jai Jai Shri Radhe!
Hare Krishna!