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all user accounts provided in the interests of harm minimisation*
A couple of less common ones that are enjoyable:
LSD + Mushrooms - trip ABSOLUTE balls. Open eye hallucinations – seeing little men running around, symbols, patterns – your brain is just overloaded with a new sense of reality. Definitely recommended if you like to trip hard. No cross tolerance, they add together and spank you hard. Pulling nitrous is the most beautiful and profound experience, verging on indescribable.
5MeO-Dipt + MDMA - The 5 MeO makes you very "jittery" and combined with MDMA gives massive body rushes. Usually mix around 8-10mg + half a pill, but that's enough to have very intense choppedness. Nausea is a definate problem with 5MeO, but if you can work through the body load it becomes kinda pleasureable. Apparently very "sexual" for some, while others are trapped between diarrohea and vomitting (akin to DXM bodyload).
DXM + Nitrous + Weed - At higher plateaus (eg 10/11 mg/kg) i find this combination not really that "pleasureable", more a way to fuck yourself up seriously and re-create brain damage. You get dissociation (ie you legs and arms feel like they're floating above you, you can see the room from 6 inches above your head), your tactile sensations change (eg cloth feels like water, water feels warm and furry) and the "hangover" last a good 2 days (if not longer). This combination also has the real bad side effect of extended impotence. A trip to build character rather than to "have fun".
MDMA + GHB/1,4B - Usually take g or 1,4B after the peak of the pill is wearing off. Have to be careful with dose as just a little too much g can make you pass out and miss the good stuff. The effects intensify the pill, make your body feel fantastic, your brain goes crazy and you will fuck anything, anywhere, anyhow. Good fun near running water and naked bitchez

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Alcohol + Cocaine - extremely good synergy. A good mindset, the lines taste nice, and it's oh so ASX/fashion shoot.
Alcohol + LSD - go extremely well together. The alcohol makes you laugh more, relaxes you more (allowing more hallucination) and is just generally awesome fun. Really hard to hold it together for public activity at high doses though. A good combo for heading to the casino hehe.
MDMA + 2CB - the 2cb just opens up a whole new chapter of hallucinations from MDMA. Tracers, laser beams splitting, stretching and bending, swirling patterns of colours, it adds heaps to the experience. Had one of my best nights out ever on this combo.
MDA + LSD - noticeably different to a normal candyflip. The "emotive" aspect of MDMA is definately lessened, and there are more intense visuals. An extremely good combo for listening to psy and watching the ceiling. It also lasts nearly 2 hours longer than you'd get outta MDMA.
Weed + Cocaine - the effect of a couple of lines after being stoned is enjoyable. Clears your head up immensily, makes you buzz hard, and the drip feels nice. The good morning combo.
Meth + MDMA + K + LSD + GHB - It can be done, and it is exttemely enjoyable. Very messy: your body is getting pulled all sorts of different ways. This combo can really push your body's limits, so make sure you have an in-depth knowledge and a high degree of comfort with all chemicals involved, and with your body's limits.
There are way more combos, it's interesting to hear other people's and whether they enjoyed them.
But, please if you read this thread and see stuff you haven't tried or didn't know existed, please don't rush in, start on a low dose and research as much as possible. I have friends who've taken DXM and MDMA, I've had Alcohol + GHB, etc - extremely dangerous combos that could have killed us because we didn't research enough or thought that we would be indestructible.
There are plenty of synergies to explore, but remember your safety is a more important consideration than going hard. *Stares jealously at horse* hehe :S