Tramadol Quinidine

Inhibition of the hepatic metabolism of tramadol to morphine by quinidine may reduce its opioid effects (SEDA-21, 90). 

Metoprolol Midazolam Naproxen Nifedipine Omeprazole Ondansetron Orphenadrine Phenytoin Piroxicam Progesterone Propafenone Propranolol Quinidine Rifampin Sibutramine Sildenafil Simvastatin Tacrine Tamoxifen Terfenadine Testosterone Theophylline Timolol Tolbutamide Tramadol Triazolam Verapamil Warfarin (R) Warfarin (S) Zolmitriptan Zolpidem... 

A placebo-controlled study in 12 healthy subjects found that quinidine 50 mg had virtually no effect on the analgesic effects of tramadol 100 mg but it inhibited its effect on pupil size. The manufacturers say that quinidine increases the tramadol AUC and maximum level by 25%, but this is within the normal pharmacokinetic variation seen with tramadol. Tramadol is partially metabolised to the active metabolite O-desmethyltramadol (which affects opioid receptors), by the cytochrome P450 isoenzyme CYP2D6, and it is this enzyme that is inhibited by quinidine. Blockade of the production of this metabolite appears to have little effect on the analgesic effect of tramadol. No special precautions seem to be needed. 

The interaction between codeine and quinidine is well established and clinically important. Codeine will be virtually ineffective as an analgesic in extensive metabolisers of CYP2D6 (most patients) taking quinidine. An alternative analgesic should be used (possibly dihydrocodeine, see below, or tramadol, see Opioids + Quinidine , p.l83). No interaction would be expected in poor metabolisers (about 7% of Caucasians), but codeine is probably unlikely to be effective in these patients in any case. Whether the antitussive effects of codeine are similarly affected is not established, but it seems likely. Note that this interaction has been used clinically in an attempt to treat codeine dependence. ... 

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