Credits: I'd like to thank steppa for suggesting that I write this FAQ. It's not exactly what you asked for, but I hope you like it. 
By posting this information, I recognize that it could potentially affect people in a negative way. It is possible. Aromatherapy teaches to respect all oils and plants used, and discourages the use of scents for intoxication. But I'm not a purist. I have some specialized knowledge from years of aromatherapy experience, but I also use oils and other substances for transformative, transcendent experiences, or to just plain old get trashed sometimes. There has been TONS of misinformation on this and other internet forums about essential oils and their proper uses. Therefore, I'd like to give my knowledge so that everyone can discern for themselves if essential oils can be allies. I put this into a FAQ layout because the same questions seem to keep coming up.
Before you start, are essential oils going to make me see elves/blast into hyperspace/melt my face off/make me see snakes coming out of my shower head?This is very unlikely. Essential oils are very different things than DMT/LSD/other psychedelic compounds, so don't expect to fly out of your mind. They are powerful, though, and if you are interested enough to read on I'll explain what they can and can't do.
What are Essential Oils?Taken from Wikipedia:"An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils, or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove. An oil is "essential" in the sense that it carries a distinctive scent, or essence, of the plant. Essential oils do not form a distinctive category for any medical, pharmacological, or culinary purpose." Hydrophobic is key here - it's very hard to get effects from ingestion without a delivery system, which beyond the scope of this FAQ.
Why Essential Oils?Most compounds of interest in plants are contained in the essential oil. It is more efficient to use 3 drops of rose oil than a pile of roses. It is more compact and efficient for transport.
Are Essential Oils Safe to Use for Intoxication?Some oils are safe as intoxicants, and some DEFINITELY are not. Like all things in life, it depends on a number of factors. The number one factor from a pharmacological perspective is, what's in the oil? As an example, oil of wormwood contains thujone, which is a neurotoxin; wormwood is also a principle ingredient in absinthe. The essential oil is much more dangerous than absinthe is. It is more concentrated, has more thujone, and could cause harm if not used properly. So in that example I would say, no it is not safe to try and trip out on oil of wormwood. However there are other oils which have compounds in them that are very safe AND help change your perspective, alter color perception, give a feeling of universal love, etc.
I've read that the best way to take essential oils is PO (by mouth). Is that a good idea?It is almost never a good idea to ingest essential oils. There are two main reasons that this is my position. The first is that the oils are extremely concentrated and have caused or contributed to kidney failure in a few cases; granted these individuals, to my knowledge, had some existing conditions, but it still happened and we should exercise caution. You have to know what you're doing to ingest the oils. There are safer ways to do it then just chugging down your bottle of oil of nutmeg or oil of elemi or whatever the kids are doing these days, but because of reason number two I'm not going to discuss what those safer methods are. Reason number two is that it's a huge waste of product. Essential oils can be expensive, especially if they are specialized or obtained from a small herb shop. Your GI track is the worst place to absorb the active compounds in essential oils because they are so hydrophobic. For those of you that have tried ingestion, remember how you started to have strange tasting burps/strange smelling gas very soon after you drank it? That's because your stomach is sending enzymes and stomach acid to get the oil out of you before absorption; it's a defense mechanism. If you drink a 5mL bottle of oil, and nothing happens, that's a waste. You could have used 3 drops of oil in a diffuser and just sat there inhaling the vapors for a while, and most likely that would have worked much better.
But I'm not into aromatherapy. I just want to get high.A lot of people say things like that to me when I talk about essential oils. The thing is, most people today in modern society have no exposure to actual plant based scents. Most of the scents we use every day are synthetic reproductions of traditional essential oils, because it's much cheaper to get compounds from an aroma factory than it is to select proper ingredients, distill them for their oil, and then package the oil properly after testing for purity. For instance, everyone is familiar with "lemony" smells, but few people have hung out inhaling oil of lemon, obtained from lemon peels. Oil of lemon has been tested as an antidepressant with good results in at least one published trial; there is no antidepressant property in lemon scented dish soap because the smell is synthetic.
And about getting high, did you know that the terpene fraction of cannabis is just as important as the THC content? Terpenes are ubiquitous in nature, and are primary active ingredients in many essential oils, including oil of cannabis. The difference in strains and their effects is mostly due to differing terpene profiles. When the DEA raids a pot farm and seizes the goods, sometimes the officers complain that the smell got them high; this is from the terpenes being emitted from the plants. Anyone who's taken straight THC pills knows that it's not nearly as good as smoking good herb. So, if you've ever appreciated good, kind bud, then you already ARE into aromatherapy, AND YOU DID IT TO GET HIGH.
Okay, now I'm interested. What is in cannabis that I can get from other essential oils?Read this article about terpenes in the cannabis plant. The main terpenes that I would focus on to start are pinene, limonene, myrcene, and linalool. There are more, but these make up a large fraction of the terpene profiles in most popular cannabis strains, and they are easy to find in the essential oil aisle at your local herb shop/health food store/chain store like Whole Foods. These terpenes all have very low toxicity.
Pinene comes in two forms, alpha and beta. For the most part, the two biggest effects of the pinene compounds are (1) mental stimulation and (2) bronchiodilation. Limonene primarily produces an antidepressant effect via 5HT-1A stimulation. Myrcene is an analgesic and muscle relaxant/hypnotic, and is a big component of good weed; it mellows out the mood. Linalool is a sedative, local anesthetic, and anti-anxiety agent; it's effects have been well documented outside of the above paper, and a recent published study (2012) showed linalool as an effective treatment for migraine headaches.
OK, so we have these terpenes to play with, now comes the fun part which is figuring out the right mix for what you want to feel. This is basically stoner science, and it's a lot of fun. I've personally given up on regular stimulants - I don't drink caffeine any more, and I don't take any harder stimulants either. They don't really agree with me. So I like to stay away from too much pinene because stimulants usually make me nauseous. However, the mix needs some pinene because we need bronchiodilation to better absorb all the other actives when we inhale the vapor. I like the three other terpenes, and depending on the mood, I'll mix theme to different degrees. For more of a euphoria, and an upbeat experience, I go heavy on limonene. For a chill day, I go heavier on myrcene. For really relaxed experiences or to help me fall asleep, I go heavy on linalool.
For me, the 3 essential oils I would use in this stoner's mix would be: oil of lime, for pinene and limonene; oil of lemongrass for myrcene; oil of lavender for linalool. An alternate to oil of lavender would be oil of sweet basil (different from holy basil, which is not useful for this discussion), but be careful with oil of sweet basil because it also contains methyl chavicol, which is an allylbenzene - another class of compounds which may or may not induce psychedelic states. More on this later. An alternate to oil of lime would be either oil of orange (which also contains less pinene) or oil of lemon (which contains more pinene). I think lime is best for me but you may have different feelings.
How do I administer the active components? What's the "dose"?Administration: The first step is mixing the oils in whatever proportion you desire. A suggestive guideline would be make a very small mix first and then expand it. What I mean by this is use one drop as one part in your blend. My blend right now is two parts lime, three parts lemongrass, one part lavender. Experimentation is a good thing, and each day will bring desires for a different blend - that's all fine and good, just don't make the mixture one part equals half a bottle of oil because you'll run out fast!
Now, administration. Option one: put about 3 drops of the oil in a small glass or glass jar and just let it sit out near you while you slowly and deeply inhale and exhale for a few minutes. This method works great. It is usually a little calmer and milder than option two. There isn't a good way to save the oil when you're done, unless the glass jar has a lid with a really tight seal; even then, it will probably evaporate into the headspace while it's closed and maybe work one more time. But it's easy and it works so start here.
Option two: make an essential oil scented trip candle. This works even better, and the good part is that the essential oils will get trapped in the wax while it isn't burning, allowing you to keep the candle and use it multiple times. There are different ways to make scented candles (a quick google search will do the trick), but the easiest way for our purposes is a quick fix to a tea light. When I say tea light, I mean the very small round candles that come in little metal dishes. The metal dish gets hot fast, which turns wax from solid to liquid phase quickly. To make the trip candle the safe way, burn the candle for a few minutes until a pocket of liquid wax has formed; this usually takes about 5-10 minutes. Now, blow out the candle, and quickly take your mixture and add a bit to the liquid wax. To start, I'd do about 5 drops of the mixture; I usually use about 15-30 drops total, after there is a sizeable pocket of liquid wax. But that is probably overkill and a waste of product, especially if this is your first time. So start conservatively and use about 5 drops. From a regular 1mL dropper that usually comes to about 0.2mL.
Which brings us to the topic of dosing. How do we know how much to use? What's a threshold dose of these terpenes? It's a difficult question to answer. Scientific information is very limited on this subject, so this is where I will diverge from science for a bit and focus on subjective experience. With essential oils, I find that there is a sort of "breakthrough" experience where you suddenly realize that you are high/tripping. It is a very different experience from most drugs that you may have taken in the past. Most likely, you won't feel fuzzy headed, and you will almost certainly NOT see elves or other universes or a cosmic supernova reflected in a drop of rain. However, you may feel: universal love; peace; enhanced perception of color; enhanced emotions, both good and bad; connection to nature; a strange sensation that I call "merging" (I made it up so I could explain it to people once and then just use the word, but I like it) where it sort of feels like you "are the oil", or that you have bonded with the oil; other sensations that are hard to describe but are noticeably different from normal life. You will also notice that, once you "break through", your sense of smell will be super sensitive, and you may have a synesthesia experience where you can feel or see smells - it's very weird the first time, and I find that oils high in limonene tend to do this for me more than the others.
How long until I "break through"? That's also hard to answer. When I first smoked cannabis, it did nothing for me, and this continued about 6 times. The seventh time, I "broke through" and got so high that I felt like I was transported to a magical land where everyone was happy and thoughts flew in and out of my mind. You may have similar experiences with essential oils. But there is a lot of evidence from very old sources (we're talking Middle Ages here) that very few people have no effect from essential oils in general. You may not react to every oil the same way as someone else does, but it will most likely effect you when you reach a threshold dose.
My recommendation on dosing, with either method: inhale the vapors until something happens, then either cover the little glass or blow out the candle and see what happens for a few minutes. Do I like this? Do I want more? Do I need fresh air? Listen to your body, and be objective. Don't be a hero and do an hour trip candle session your first time out - it might not agree with you, you might pass out with a flame going, etc ... follow the wisdom of aromatherapists and respect the plants, and go slowly.
How long does this "stoner mix" affect you? What's the normal time table?This depends on the amount taken, but usually after administration ends and I leave the room/house, I'll feel the effects for 2-4 hours, with plenty of fresh air. Staying in this environment may prolong the effects. Another trick to prolong the effects is to come and go from the administration area; each time you come home, the effects will intensify, and when you leave they will begin to taper. To clean out the air in the room, open a window and turn on a fan to get clean air in the room - otherwise you might get sick from inhaling too much vapor. If you want to administer outside, put a few drops of your mix into a small vial with a tight cap, then remove the cap and take some deep breaths when you want your dose. Repeat as necessary. A small glass vial usually sells for about $2.00 in my area, you'll probably find similar prices near you.
How do I store my mix?In a well sealed glass container. Do not use plastic because many essential oils will act as solvents and may dissolve some plastic into your mix.
I've heard a lot of talk about oil of nutmeg and oil of elemi on this and other internet forums. Why haven't you mentioned these? Are there other good oils for getting high/tripping/changing mental states?I haven't mentioned nutmeg or elemi because they are more expensive, and until now I've been talking about basic terpenes that work in pretty much everyone. The actives in nutmeg and elemi (probably) are allylbenzenes. Allylbenzenes are not terpenes; they are phenylpropanoids. These compounds also abound in essential oils. Basically, there is debate as to whether or not these compounds are active as is, or if there is some sort of biotransformation that must occur first in order to activate them for our purposes. The supposed nutmeg and elemi active compound, elemicin, has shown on this and other forums, and in some literature, as being hard to activate and various effects have been reported which are vastly different, ranging from being tired to full out hallucinations.
Allylbenzenes, in my experience, are active compounds when inhaled, but usually they need to be tempered with terpenes to make the experience enjoyable. This poses something of a problem when trying to discuss what the effects of say elemicin are. If I administer elemicin with limonene, for example, and I feel euphoria, how do I know if it's just the limonene? Subjectively, I feel differences when adding new compounds, but I've been doing this for a while and I notice certain subtleties and differences from past experiences. To start out, I'd say let's steer clear of this stuff for the time being.
Advanced: I want to trip on elemicin and other allylbenzenes! Give me the scoop!Again, you are NOT ... I repeat, you are NOT going to see elves or infinite light in a ray of sun. You may experience random thoughts or images, but they will not look "real" per say, meaning you will know that it's just fantasy and nothing more. I've noticed pattern recognition increase dramatically. I didn't see any differences in color perception. Personally I felt jazzed up, and my mind raced a bit with random imagery about mostly insignificant content, and pattern recognition. That's pretty good for a legal, essential oil - but you need to have reasonable expectations. If DMT is the only thing you like, elemicin will not fill the void.
The thing is, just smelling elemi oil probably won't make you trip out of the box. You need to break through first, then you can feel the effects of the allylbenzene that you're trying to activate. I would avoid the sedative oils and focus on a little pinene and a decent dose of limonene. Be patient and get to the point where you start to feel the shift in your mental space. Relax, and feel happy. Then, take out the allylbenzene oil. Put a few drops in a small vial or dish or whatever you use for administration method one above. Slowly inhale the new vapors. Be receptive, let the plant speak to you. You may feel an instant and sudden change in the mood - don't freak out! Embrace it. It's going to be ok. You can always cover the vial and leave the room and get some fresh air outside to calm the senses. Bring some lemongrass oil with you, as it usually has the effect of chilling out a harsh trip and adding an element of love and connectedness.
Are allylbenzenes going to blow your mind? Like any substance used for spiritual or recreational uses, this is going to depend on set and setting, and other factors outside of your control. It might not really work for you one day, and it might completely blast you off another day (except without the elves!). Experimentation is good, but at the end of the day, temper expectations and enjoy what happens at that moment. As the timeless words on the t-shirt says, "It's All Good."
Advanced: I've heard that you can boost the effects of cannabis with essential oils. Is that true?Yes! If you spike your weed with terpenes, you'll definitely feel the effects more. The classic one to use is oil of lemongrass, but lavender, citrus oils, and even others like carrot seed oil can boost the effects of the weed. It is outside the scope of this FAQ to go into detail,
but check out the grasscity forum where there is a lot of information on cannabis boosting with oils.