Coadministration of bupropion (150 mg) and ritonavir (100 mg twice daily) decreased bupropion AUC by 22% and Cmin by 21%. Coadministration of ritonavir (600 mg twice daily) decreased bupropion AUC by 66% and Cmin by 62%. Bupropion is primarily metabolised by CYP2B6. Concurrent administration of bupropion with repeated doses of ritonavir is expected to decrease bupropion levels. These effects are thought to represent induction of bupropion metabolism. However, because ritonavir has also been shown to inhibit CYP2B6 in vitro, the recommended dose of bupropion should not be exceeded. In contrast to long-term administration of ritonavir, there was no significant interaction with bupropion after short-term administration of low doses of ritonavir (200 mg twice daily for 2 days), suggesting reductions in bupropion concentrations may have onset several days after initiation of ritonavir co-administration.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Concurrent administration of bupropion with ritonavir may decrease plasma levels of both bupropion and its active metabolite (hydroxybupropion). Patients receiving ritonavir and bupropion concurrently should be monitored for adequate clinical response to bupropion.
Norvir Prescribing Information, AbbVie Inc December 2016.

The effect of ritonavir (200 mg, 4 doses) on a single dose of bupropion (75 mg) was investigated in 7 HIV-negative males. The effect on bupropion exposure was inconsistent and highly variable (20 ± 20% increase in AUC, 31 ± 21% increase in Cmax, mean ± SE). The only statistically significant changes in PK parameters of bupropion and metabolites were a 14% decrease in threohydrobupropion AUC and an increase in Tmax for erythrohydrobupropion from 2.6 to 5.2 h. These findings indicate that short-term ritonavir dosing has only a minimal impact on a single dose of bupropion; however, further studies are required using multiple doses over a longer period. Ritonavir has minimal impact on the pharmacokinetic disposition of a single dose of bupropion administered to human volunteers. Hesse LM, Greenblatt DJ, von Moltke LL, Court MH. J Clin Pharmacol, 2006, 46(5):567-576.

Bupropion metabolism is mediated primarily by CYP2B6 and in vitro data has pointed to nelfinavir, ritonavir and efavirenz being inhibitors of this enzyme. There is therefore potential for a drug interaction. This report is a case series aimed at documenting the clinical consequence of the co-administration of bupropion with nelfinavir, ritonavir, and efavirenz. The median duration of concomitant use was 8 months and ranged from 3 weeks to 2 years. There were no episodes of seizures documented in any of the medical records for patients who took bupropion with the CYP2B6 inhibitors. However, these data should be regarded as highly preliminary since: there is a low rate of seizure incidence in the literature, There were no PK measurements which are vital if we are to understand this interaction and no patients were receiving ritonavir at a dose above 100 mg bd. Further data are therefore required.
Concurrent use of bupropion with CYP2B6 inhibitors, nelfinavir, ritonavir and efavirenz: a case series. Park-Wyllie LY & Antoniou T. AIDS, 2003, 17: 638-640.

The hydroxylation of bupropion was studied in vitro using human liver microsomes. It was concluded that ritonavir, efavirenz and nelfinavir inhibited CYP2B6 and therefore bupropion hydroxylation. The IC50 for ritonavir was 2.2 µM. This suggests that there is the potential for clinical drug interactions.
Ritonavir, efavirenz, nelfinavir inhibit CYP2B6 activity in vitro: Potential drug interactions with bupropion. Hesse LM, von Moltke LL, Snader RI, et al. Drug Met Dis, 2001, 29:100–102.