tramadol is a really messy drug. it actually is an active prodrug -- tramadol itself is not an opioid, it is a norepinephrine (aka nor-adrenaline in euro medicine) reuptake inhibitor (NRI). meaning it also functions as a peripheral stimulant. it is only once the tramadol hits your liver and a carbon atom is removed from tramadol's oxygen, forming O-desmethyl-tramadol, that the drug becomes an opioid.
there is a rate limit at which that process can happen. this is part of why the pharmaceutical industry calls tramadol a "non-narcotic," (even though it is definitely a narcotic)... past a certain point, taking more tramadol just increases the NRI effects without increasing the opioid effects.
this is also why tramadol lowers the seizure threshold. unlike traditional opiates, tramadol greatly increases the chance that an individual will experience an epileptic seizure. particular if they're abusing the stuff, but it is not unheard of for people using it for chronic pain to end up having a seizure disorder.
also, tramadol doesn't play too nicely with a lot of other medicines. combined with the wrong things (certain stimulants and serotonergic drugs), it can cause hypertensive crisis. and tramadol is metabolized by the liver enzymes CYP3A4 and CYP2D6, two of the most common enzymes that pharmaceuticals are broken down by. specifically they take the outlying carbon off of methoxybenzene and (i think) methylaminobenzene style groups, which are present in a lot of pharmaceuticals because they are chemically easy manipulations to make when trying to discover new drugs. the fact that tramadol competes for these enzymes with many other drugs means that the rate at which the tramadol -> o-desmethyltramadol conversion happens can be greatly reduced in individuals taking multiple medications. leading to that individual getting most just a NRI effect and not a opioid pain relieving effect from their tramadol. this can cause the NRI tramadol to build up in your system over time, too, adding to the risk of the seizure threshold being lowered.
the pharmaceutical industry is pushing really hard to make tramadol the next big thing, and it kind of already has become it. but i think the stuff is awful medicine, and if it was up to me it wouldn't be on the market.
if doctors educated patients on the proper way to use traditional opiates, and the risks of taking them long term, we wouldn't have a need for pro-drug painkillers.