From the DXM FAQ that can be found on Erowid:
"The first plateau is probably the hardest to hit; many people "overshoot" it. Please keep in mind that these effects listed are general effects, and that individual results may vary considerably.
A general narrative of the first plateau can be constructed. At about 30 minutes to 1 hour after dosing, an "alert" sensation is noticeable; this is simply a feeling that is unique for individual and signals the begin of altered consciousness. The experience has only a vaguely "drug-like" character for about 10 minutes, after which restlessness and slight stimulant effect are noticeable. After another 10 minutes or so, movement and position sense are altered; those with motion sickness begin to notice nausea. Gravity starts to feel weird, and one may bounce around a lot. Emotions may start to become intensified. There is a slight feeling of dissociation from reality, but overall the experience is slightly intoxicating, with intensified emotions and sense of importance from everyday events. This effect peaks and then slowly subsides until it is unnoticeable.
A first plateau trip usually takes between 20 and 40 minutes to start (on an empty stomach), peaks about 1.5 to 2 hours later, and lasts between 4 and 6 hours. Gel capsules take up to 1 hour additional to dissolve. Hangovers are very rare from this plateau, but if they do occur, they tend to consist mainly of lethargy.
The primary effects of the first plateau are general euphoria, euphoria specifically linked to music and motion, slight disturbances in balance, moderate stimulation, and very slight intoxication. The intoxication and balance disturbances are similar to that induced by alcohol, but much weaker and without the mental confusion; there is little if any mental sluggishness or confusion with a first plateau trip.
Some people have difficulty hitting the first plateau. It can take several trials; as a general guideline, if you notice double vision, you've gone way too far. A lot of the more pleasurable first plateau effects, in particular the music euphoria, are set and setting dependent. Being in good physical condition, avoiding excessive caffeine, and being in a good mood are all important factors in achieving a good first plateau dose.
Positive first plateau experiences are one of the first to go with regular use. Part of this seems to be tolerance (which builds quickly and lasts for considerable time). Another part seems to be a familiarity with the first plateau experience; after awhile it no longer seems quite so profound or interesting. Some have suggested changing set and setting as a way of regaining the more interesting aspects of the first plateau.
With the second plateau (around 2.5-7.5 mg/kg, 6.4 mg/kg suggested from regular users) several new effects become evident. The most profound is that DXM begins to take on a heavier "stoning" characteristic, and senses and cognitive function are affected accordingly. Closed-eye hallucinations start for some people on the second plateau. Some of the first plateau effects, e.g., the music and motion linked euphoria, may diminish or stop entirely.
Second plateau trips usually take between 30 and 60 minutes to start (on an empty stomach), peak about 2 to 3 hours later, and last about 6 hours. Again, gel capsules take up to 1 hour additional to dissolve. Hangovers are not common with lower second plateau trips, but some people experience them.
A general narrative of the second plateau: The trip beings identically to the first plateau trip, although the alert and early effects may be noticed earlier. After passing to the music and motion euphoria stage, the first plateau sensations begin to be overshadowed by disruptions in sensory processing, as sensory input begins to get "choppy". Both sight and sound take on a dreamlike characteristic and one begins to feel increasingly detatched from the outside world. There may be bursts of sensory deprivation where the outside world seems to go away, but overall one maintains contact (somewhat incoherent) with the outside world. After a few hours of an overall "stoned" feeling, the sensations begin to subside. A slight hangover consisting of lethargy may be noted the next day."
), and I would like some general descriptions for 1st and 2nd plateau effects.
