To evaluate polypharmacy, anticholinergic burden (ACB) and drug-drug interactions (DDIs) in people with four-class-resistant HIV (4DR-PWH). We performed a cross-sectional study, including 4DR-PWH from the PRESTIGIO Registry taking at least one non-antiretroviral drug. Polypharmacy was defined as taking five or more non-antiretroviral drugs. ACB was calculated using the ACB scale: 0 = no AC effect, 1-2 = low/moderate risk, ≥3 = high AC risk. Participants' characteristics by ACB score were compared using the Kruskal-Wallis test, and Spearman's correlation coefficient was used to assess linear relationships. DDIs were evaluated using the Liverpool database. Overall, 172 4DR-PLWH were evaluated: 75.6% males, median age 49.9 years (IQR = 45.6-56), 62 (27.1%) on polypharmacy, 124 (72.1%) using a boosting agent and 72 (41.8%) with four or more antiretrovirals. Based on ACB, 128 (74.45%), 33 (19.2%) and 11 (6.4%) had a no, low/moderate and high AC risk, respectively. The most common AC drugs were β-blockers (12.2%), diuretics (8.7%) and antidepressants (8.7%). The high ACB was significantly related to the number of drugs/person (r = 0.33, P < 0.0001) and the number of clinical events (r = 0.222, P = 0.004). Overall, 258 DDIs were found between antiretrovirals and co-medications in 115 (66.8%) PWH, and 14 (8.1%) PWH received contraindicated drug combinations. In 4DR-PWH, polypharmacy, DDIs and the proportion of people with moderate/high AC burden were high. In 4DR-PWH undetectability achievement and maintenance is the priority and use of boosted PIs is common. A strict collaboration (infectious diseases specialists, virologists, pharmacologists) is needed to limit the risk of ACB and DDIs and to explore the advantages of new antiretrovirals.