LHPG Comment: Note the different emphasis in SPC (European) and Prescribing Information (USA). In this case the charts reflect the use of low dose ritonavir as a pharmacokinetic enhancer to boost plasma concentrations of other protease inhibitors. The contraindication in the SPC is more applicable to ritonavir when given at full dose (600 mg twice daily).

The use of meperidine and ritonavir is contraindicated due to the increased concentrations of the metabolite, normeperidine, which has both analgesic and CNS stimulant activity. Elevated normeperidine concentrations may increase the risk of CNS effects (e.g., seizures). Coadministration of pethidine (50 mg single oral dose) and ritonavir (500 mg twice daily) decrease pethidine AUC and Cmin by 62% and 59%, respectively, but increased normeperidine AUC and Cmin by 47% and 87%, respectively.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (500 mg every 12 hours for 10 days) with a single dose of meperidine (50 mg oral dose) caused a 62% and 59% decrease in meperidine AUC and Cmax, respectively (n=8), and a 47% and 87% increase in the normepridine metabolite AUC and Cmax, respectively (n=6). Dosage increase and long term use of meperidine with ritonavir are not recommended due to the increased concentrations of the metabolite normepridine which has both analgesic activity and CNS stimulant activity (e.g. seizures).
Norvir Prescribing Information, AbbVie Inc, December 2016.

The effect of ritonavir (500 mg bd) on the pharmacokinetics of pethidine (meperidine; 50 mg oral) was studied in 8 healthy volunteers. In the presence of ritonavir, the AUC of pethidine decreased by 67% and Cmax decrease from 126±47 to 51±21 ng/ml. The AUC of the metabolite (norpethidine/normeperidine) increased by 47%. Based on these findings, the risk of narcotic-related adverse effects from this combination appears to be minimal. However, the increased concentrations of the metabolite suggest a potential for toxicity with increased dosages or long-term therapy.
The effect of ritonavir on the pharmacokinetics of meperidine and normeperidine. Piscitelli SC, Kress DR, Bertz RJ, et al. Pharmacother, 2000, 20:549-53.