Apparently mixing opiates with Gaba-based medications can decrease the effects of the opiates and enhance the effects of the Gaba-based medications.

Below is a summary of some information from a drug research site:

When Neurontin (gabapentin) is taken along with hydrocodone (a.k.a. Vicodin), it decreases the bioavailability of the hydrocodone by up to 22%; while the bioavailability of Neurontin (gabapentin) is increased by 14%.

When the two types of medications are taken with a time frame between the doses the interactions of the medications can be emphasized or altered in unpredictable ways. If an opiate (such as morphine) is taken first and then the Neurontin/Gaba-based medication is taken a couple of hours later, the opiate will be on its way to reaching peak plasma levels when the Neurontin is just entering the bloodstream. That scenario can leave the opiate's effects unchanged, but the Neurontin (gabapentin) bioavailability may be increased by as much as a whopping 44%.

Similarly taking naproxen (a.k.a. Aleve) increases Neurontin's bioavailability by 12-15%, while there is no apparent affect on naproxen's bioavailability.

It is uncertain if this type of bioavailability effect carries over to the artificial opioids, like Demerol or buprenophine.

As each person's reaction to Gaba-based and pregabapentin medications is highly variable when these medications are taken by them selves, combining them with opiates has the potential for making a real mess of things.
That isn't to say that they can't be taken together (fairly often they are), but one needs to be supervised fairly closely, at least at first by an MD/pharmacist.

Overdose symptoms of Neurontin and other Gaba-based medications is a strong possibility when combined with opioids; and the first sign of this would be enhanced CNS depression.
It is interesting to note that some doctors refer to Neurontin by its rather unkind nickname (morontin) which describes its general mind slowing effect.

Antihistamines might have the same or similar effect on bioavailability when taken together with the Gaba-based medications.
It appears that a great deal of caution should be used in combining the Gaba-based medications with anything.