It's interesting why the ER has this effect (I noticed it also---I crushed the pill with my teeth and it was a rotten taste but I got used to it). I suspect it is due in part to the racemic mixture used: one enantiomer has a 4 fold greater affinity for the mu opioid receptor than the other but the combined affinity of the racemic mixture is 1/6000th of morphine to the mu opioid receptor. And in part (consequently?) due to the rate at which this racemic mixture is converted into its major metabolite (M1) O-desmethyltramadol which has a 30 fold potency over the (+)- tramadol.
O-desmethyltramadol is 1/200th the potency of morphine in terms of binding affinity to the mu opioid receptor, whereas the racemic mixture of "tramadol" is 1/6000th. The former also as a longer half-life of 9 hours as opposed to 6 hours for tramadol itself. The M1 metabolite at any point represents 10-20% of the tramadol in normal metabolisers (i.e., without a defective CYP2D6). So if you take 400mg of tramadol, the maximum daily recommended dose (for IR I think this is fine), you have about 40-80mg of the M1 metabolite, which equals to about 0.2% to 0.4% of a mg of morphine?!? So you can see why in general it has a low abuse potential, and for those who manage to get the CYP2D6 going (there are inducers) you can feel more euphoric and perhaps consequently get addicted. Who knows?
This is all old hat, and can be gotten from the Wikipedia page on tramadol (
http://en.wikipedia.org/wiki/Tramadol). I just wrote about that to summarise what I think is interesting about tramadol. I'll leave with this final quote (one of the few sentences there without a citation): "The extended-release formulation of tramadol—which, amongst other factors—was intended to be more abuse-deterrent than the instant release) allegedly possesses more abuse liability than the instant release formulation."
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I myself have never really euphoric on tramadol IR (I've tried a variety of brands). But I did on the ER crushed with my teeth. Hmm.
Also I am not sure 100% sure, but if you look at the seizure reports on Erowid, especially at low doses, it is Ultram that is causing the problem. At high doses it doesn't seem to matter. Thus the 400mg limit. It actually starts around 500mg but below 500mg it seems to be aggravated by another factor. Males tend to ingest higher doses and consequently tend to report higher seizure rates than females.
I have had one seizure I can attribute to tramadol (I was pushing the 400mg limit---I may have taken only 400mg that "day" but I might've exceeded that by 50-100mg if you count it as a 24 hour period---I really wasn't keeping careful track is my point). This came after one seizure I can attribute to hyponatremia caused by drinking too much water only (due to what?

. The first two seizures were a year apart. Before both seizures I had stopped drinking alcohol for several months (though I was physically addicted to alcohol). A year later approximately I had one more seizure, this time I was drinking a LOT. I then drank 24/7, had all kinds of things happen (including seizures), went to detox and treatment and have stayed clean from alcohol for 3 years. Never want to touch it again. I've also been on a low dose of carbamazapene for 3 years and have not had a single seizure (crossed fingers).
I actually found the ER 100mg capsules, once you crush them up and parachute them at least, to provide a much nicer high than the 50mg IR pills. Basically same effects, but the ER seems to feel a bit "cleaner" if that makes any sense. Like, they're just as stimulating, if not a little more so, than IR, but it's a very subtle stimulation that really lends itself well to exercise. And the opioid feeling also seems a bit stronger, if more subtle as well.
I'd take 400mg of the crushed up ER over 400mg of the normal IR any day.
Technically, I do believe 2x 50mg IR would be the equivalent of 1x 100mg ER (assuming you crush it up; if you don't crush it up the different is obviously that it will take longer to fully effect you and the effects will be more mild although they will continue over a great amount of time)