This is a partial list of recreational and medical drugs which
inhibit the P450 enzymes 2D6, 3A4, and 3A5. Not all of them will
inhibit all of the P450 enzymes, but it's safe to say that a
substantial number of these will interact with DXM.
Generally speaking, inhibitors of P4502D6 include antidepressants (both SSRIs and tricyclics, and possibly MAO inhibitors), antiparasitics (especially antimalarials), antihistamines (both prescription and overthecounter), neuroleptics, beta blockers (drugs for high blood pressure), and antineoplastics (anticancer drugs). Methadone is a P4502D6 inhibitor, and it is likely that many alkaloids, especially of plant origin, may be mild to moderate P4502D6 inhibitors.
Drug 
Uses 
P4502D Enzymes 
Potency 
Ref 

ajmalicine 

2D6 
strongest 
(164) 
carbon monoxide 
poison 
2D6 

(160) 
chloroquine 
antiparasitic 
2D6 
medlow 
(172) 
chlorpheniramine 
antihistamine 
2D 
medhigh 
(151) 
citalopram 
antidepressant 
2D6 
medlow 
(166) 
clozapine 
antipsychotic 
2D6 
low 
(171) 
desipramine 
antidepressant 
2D6 
low 
(152) 
diphenhydramine 
antihistamine 
2D 
medhigh 
(151) 
doxorubicin 
anticancer 
2D6 
medlow 
(165) 
fluoxetine 
antidepressant 
2D6 
medhigh 
(152) 
fluvoxamine 
antidepressant 
2D6 
medhigh 
(152) 
imipramine 
antidepressant 
2D6 
med 
(152) 
lomustine 
anticancer 
2D6 
med 
(165) 
mepyramine 
antihistamine 
2D6 
high 
(151) 
methadone 
addiction treatment 
2D6 
med 
(162) 
moclobemide 
MAOA Inh. (reversible) 
2D6, also 2C19, 1A2 

(147) 
nortryptiline 
antidepressant 
2D6 
medlow 
(155) 
oxamniquine 
antiparasitic 
2D6 
medlow 
(172) 
paroxetine 
antidepressant 
2D6 
high 
(152) 
PCP 
recreational 
2D 

(150) 
primaquine 
antiparasitic 
2D6 
medlow 
(172) 
propranolol 
betablocker 
2D6 
low 
(156) 
quinidine 

2D6 

(148) 
quinine 
antiparasitic 
2D 

(151) 
sertraline 
antidepressant 
2D6 
medhigh 
(167) 
triprolidine 
antihistamine 
2D 
medhigh 
(151) 
vinblastine 
anticancer 
2D6 
medlow 
(165) 
vinorelbine 
anticancer 
2D6 
medlow 
(165) 
Drug 
Uses 
P4503A Enzymes 
Potency 
Ref 
7,8benzoflavone 

3A4 (activator) 

(153) 
cannabidiol 
component of marijuana 
3A 
med 
(161) 
cocaine 
recreational 
3A 
low 
(157) 
clotrimazole 
agricultural fungicide 
3A (activator) 
very high 
(154) 
cyclophosphamide 

3A 
low? 
(158) 
ifosfamide 

3A 
low? 
(158) 
ketoconazole 

3A 

(145) 
pilocarpine 
cholinomimetic 
3A 
low 
(149) 
Drug 
Uses 
P4503A Enzymes 
Potency 
Ref 
1aminobenzotriazole 

Nonspecific 
medhigh 
(159) 
chlorophyllin 
geriatric 
Nonspecific 

(146) 
general anaesthetics 

Nonspecific 

(163) 
One reference ((164)) gives the
general characteristics of P4502D6 inhibitors as
A computer model of the P4502D6 cytochrome has been constructed
((170)).
Here are a few sigma ligands you may wish to research (if you are
interested in that sort of thing):
Sigma_{1} ligands in rough order of potency:
Sigma_{2} ligands in rough order of potency:
P> A known sigma antagonist is
N[2(3,4dichlorophenyl)ethyl] Nmethyl2[1
pyrimidinyl1piperazine] butanol; a sigma_{1} selective
antagonist is (1(cyclopropylmethyl) 42'4"fluorophenyl)
(2'oxoethyl)piperidine HBr.