DXM should not be used (either recreationally or at normal dosage levels)
by people who are taking a monoamine oxidase inhibitor (MAOI, rhymes with
“wowee”) - either a prescription MAOI or a recreational one such as
harmaline. Note that there is considerable confusion among drug users about
what is and isn’t a MAOI. MAOIs include a few drugs prescribed for
depression and Parkinson’s disease, and a few rare recreational drugs
derived from exotic plant sources (harmine and harmaline, from Syrian Rue
and Yagé, for example). ProzacTM, MDMA, cheese, beer, SeldaneTM, etc., are
not MAOIs - they are things to avoid when taking a MAOI. If you are taking
a prescription MAOI you will almost certainly know, as your physician will
have (hopefully!) told you to avoid eating aged cheeses. Combining DXM and
a MAOI has been fatal (3)!
Fluoxetine (ProzacTM) is a cytochrome P450-2D6 inhibitor (39) and will
change the characteristics of a DXM trip somewhat, increasing the ratio of
DXM to DXO. Other P450-2D6 inhibiting drugs, which include many
antidepressants, will probably do the same; see Section 15.1. The duration
of the trip may be greatly extended by P450-2D6 inhibitors; some users have
reported effects lasting 12 to 24 hours past the normal duration. The
potency of DXM may also be enhanced via other mechanisms by fluoxetine
(40).
Combining DXM with the antidepressants Desyrel (trazodone) or Serzone
(nefazodone) has been reported to cause liver damage!
One user reported that combining DXM with bupropion (Wellbutrin[tm])
resulted in a prolonged (3+ day) hangover and an increase in adverse side
effects.
Fluoxetine and other SSRI antidepressants, as well as tricyclics and
lithium (and of course MAOIs) may interact with DXM to cause serotonin
syndrome (see Section 6.2.9). This condition, although rarely fatal, is not
terribly pleasant. Vascular disease may increase the chance for serotonin
syndrome with DXM + antidepressants (364), and other disease conditions may
do so as well. Some DXM users who have taken DXM while on antidepressants
have reported unpleasant reactions that sound a lot like serotonin
syndrome, so you might want to watch out. Some of the symptoms of serotonin
syndrome include muscle rigidity, confusion, diarrhea, incoordination,
low-grade fever, sweating, muscle tremor, mania, agitation, exaggerated
reflexes, and nausea.
Do not take DXM with the diet drugs phentermine, fenfluramine, or phen-fen;
this combination can also cause serotonin syndrome.
DXM should not be taken (recreationally or at normal dosage levels) with
the prescription antihistamine terfenadine (SeldaneTM). This combination
has been fatal (41). Terfenadine has been implicated in other drug
interactions, incidentally. The reason for this interaction seems to be
that terfenadine, which is normally metabolized by a P450 enzyme, induces
heart irregularities when it builds up. DXM may saturate the P450 enzymes
that normally metabolize terfenadine. Incidentally, this probably applies
to other non-drowsy antihistamines, such as ClaritinTM and HisminalTM as
well; avoid combining them with DXM.
Some people find that nicotine (cigarettes) causes severe nausea when
combined with DXM. Others have noticed a general increase in physical
discomfort and “bad trips” from combining the two. Some research has
suggested that cigarette smoke inhibits monoamine oxidase (378,379) in
which case cigarettes could greatly increase the chance of unpleasant side
effects.
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