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from the DXM FAQ

15   Appendices

used at Erowid with the permission of author William White

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15.1   Appendix 1: P450 Inhibiting Drugs

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 P450-2D6 include antidepressants (both SSRIs and tricyclics, and possibly MAO inhibitors), antiparasitics (especially antimalarials), antihistamines (both prescription and over-the-counter), neuroleptics, beta blockers (drugs for high blood pressure), and antineoplastics (anti-cancer drugs). Methadone is a P450-2D6 inhibitor, and it is likely that many alkaloids, especially of plant origin, may be mild to moderate P450-2D6 inhibitors.
P450 Inhibitors
DrugUsesP450-2D EnzymesPotencyRef
ajmalicine  2D6 strongest (164)
carbon monoxide poison 2D6   (160)
chloroquine antiparasitic 2D6 med-low (172)
chlorpheniramine antihistamine 2D med-high (151)
citalopram antidepressant 2D6 med-low (166)
clozapine antipsychotic 2D6 low (171)
desipramine antidepressant 2D6 low (152)
diphenhydramine antihistamine 2D med-high (151)
doxorubicin anticancer 2D6 med-low (165)
fluoxetine antidepressant 2D6 med-high (152)
fluvoxamine antidepressant 2D6 med-high (152)
imipramine antidepressant 2D6 med (152)
lomustine anticancer 2D6 med (165)
mepyramine antihistamine 2D6 high (151)
methadone addiction treatment 2D6 med (162)
moclobemide MAO-A Inh. (reversible) 2D6, also 2C19, 1A2   (147)
nortryptiline antidepressant 2D6 med-low (155)
oxamniquine antiparasitic 2D6 med-low (172)
paroxetine antidepressant 2D6 high (152)
PCP recreational 2D   (150)
primaquine antiparasitic 2D6 med-low (172)
propranolol beta-blocker 2D6 low (156)
quinidine   2D6   (148)
quinine antiparasitic 2D   (151)
sertraline antidepressant 2D6 med-high (167)
triprolidine antihistamine 2D med-high (151)
vinblastine anticancer 2D6 med-low (165)
vinorelbine anticancer 2D6 med-low (165)
DrugUsesP450-3A EnzymesPotencyRef
7,8-benzoflavone   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)
DrugUsesP450-3A EnzymesPotencyRef
1-aminobenzotriazole   Nonspecific med-high (159)
chlorophyllin geriatric Nonspecific   (146)
general anaesthetics   Nonspecific   (163)

One reference ((164)) gives the general characteristics of P450-2D6 inhibitors as
a positive charge on a nitrogen atom, and a flat hydrophobic region, the plane of which is almost perpendicular to the N-H axis and maximally extends up to a distance of 7.5 A from the nitrogen atom. Compounds with high inhibitory potency had additional functional groups with negative molecular electrostatic potential and hydrogen bond acceptor properties on the opposite side at respective distances of 4.8-5.5A and 6.6-7.5A from the nitrogen atom.
A computer model of the P450-2D6 cytochrome has been constructed ((170)).

15.2   Appendix 2: Neuropharmacology of Recreational Drugs

Like many phenethylamines, has mixed properties of a dopamine releasing agent, serotonin (5HT) releasing agent, and probably binding specifically to 5HT receptors (likely including 5HT2A and 5HT2C). Compare amphetamine, MDA.
Mechanism unknown; may affect membrane permeability or alter ion channel function. Functionally potentiates GABAergic activity and blocks NMDA receptors, and also has actions on other ion channel receptors.
amyl nitrite
Vasodilator; recreational effects probably derive from this effect rather than being specific to a set of receptors.
Causes a non-vesicular release of dopamine and noradrenaline by neurons which normally secrete them. May have some direct effect on dopamine and noradrenaline receptors, but this is insignificant compared to its neurotransmitter releasing effect.
Targets and binds to a specific site (called the picrotoxin site) on GABA receptors, which activates them. This is a different site from alcohol and benzodiazepines, so that if you combine any of these three, they will not compete for the same binding sites. Consequently, there is a synergistic effect, which can be quite dangerous.
Similar to barbiturates, except for two factors. First, the binding site is the benzodiazepine site on the GABA receptor. Second, when a benzodiazepine binds to this site, the GABA receptor is not immediately activated; instead, the natural action of GABA is enhanced. This is the main reason benzodiazepines are safer than barbiturates, and have different effects.
An antihistamine (targeting and activating the H1 receptor) which probably has sigma1 antagonist properties; when used in combination with pentazocine, it probably blocks the sigma activity of the latter. Rarely found. The only reason I'm mentioning it is because I heard about it in a comedy skit called "Rock and Roll Doctor" and always wondered what "blues" were (until I found out). Well, if you wondered too, now you know.
Targets and blocks an adenosine receptor, probably A2 but possibly A1. This is an inhibitory presynaptic receptor, i.e., when activated it decreases the amount of neurotransmitter released by a neuron. Thus, caffeine blocks some of this inhibition, increasing neural activity.
Targets a specific receptor (or family of receptors) designated anandamide. It is not yet known whether cannabis (actually, THC) is an agonist or antagonist at this receptor, although most think it is an agonist.
See morphine.
See caffeine.
A dopamine reuptake inhibitor; cocaine blocks the transporter which takes used dopamine out of the way. Thus, dopamine secreted by a neuron keeps activating receptors over and over. Cocaine is also a sigma1 agonist, and has blocking abilities on certain ion channels (by which it exerts its local anesthetic effects).
See morphine.
See solvents.
See morphine.
Blocks NMDA receptors at the same site as DXM and PCP.
Targets 5HT2A and 5HT2C, where it acts either as an antagonist or a partial agonist. Also has some dopaminergic activity; however, the majority of its effects are mediated through the 5HT receptors.
See cannabis.
See MDMA. Release binding spectrum is probably different, and MDA may have additional effects on receptors.
Similar to amphetamine, except that MDMA causes a nonvesicular release of dopamine and serotonin (5HT). Probably has other effects as well, some of which may be significant.
Similar to amphetamine, possibly with more dopamine release.
Targets opioid receptors mu, kappa, and delta, where it acts as an agonist. Slight differences in binding spectrum to opiate receptors exist among the various natural and synthetic opiates.
Stimulates and then blocks nicotinic acetylcholine receptors.
nitrous oxide
Seems to affect phospholipid membranes, although some effects may be mediated by NMDA and GABA channels, and other ion channels. General anesthetics are similar to nitrous oxide, although more toxic.
See morphine.
PCP (phencyclidine)
PCP and related cycloarylpiperidines block NMDA receptors, as well as having a variety of other effects, including sigma agonism, dopamine reuptake inhibition, and specific effects on an unknown cerebellar receptor.
See amyl nitrite
Psilocybin (mushrooms)
Similar to LSD; acts via 5HT2A and 2C receptors and possibly additional ones.
Quaaludes (methaqualone)
See barbiturates.
Similar to amphetamine, prescribed for Attention Deficit Disorder
See benzodiazepines.
See barbiturates.
Same general theory as alcohol and nitrous oxide, but considerably more toxic to neurons (and the liver).
See benzodiazepines.
Targets and blocks alpha2 adrenergic receptors. These are autoreceptors, which normally limit the activity of adrenergic neurons. By blocking alpha2 receptors, yohimbine increases the activity of these neurons.

15.3   Appendix 3: Other Sigma and NMDA Ligands

Here are a few sigma ligands you may wish to research (if you are interested in that sort of thing):

Sigma1 ligands in rough order of potency:
  • (+)-pentazocine
  • haloperidol
  • DTG
  • (+)-3-PPP
  • DXM
  • (+)-SKF 10,047
  • (+)-cyclazocine
  • (-)-pentazocine
  • PCP
  • (-)-SKF 10,047

Sigma2 ligands in rough order of potency:
  • DTG
  • haloperidol
  • (+)-3-PPP
  • (-)-pentazocine
  • PCP
  • (+)-pentazocine
  • (-)-SKF 10,047

P> A known sigma antagonist is N-[-2-(3,4-dichlorophenyl)ethyl]- N-methyl-2-[1- pyrimidinyl-1-piperazine] butanol; a sigma1 selective antagonist is (1-(cyclopropylmethyl)- 4-2'4"-fluorophenyl)- (2'-oxoethyl)piperidine HBr.