OBJECTIVES: We quantified concomitant medication (CM) polypharmacy, pharmacokinetic (PK) and pharmacodynamic (PD) interactions, adverse effects and adherence in Australian adults on effective ART. DESIGN: Cross-sectional. METHODS: Patients recruited into a nation-wide cohort and assessed for prevalence and type of CM (including polypharmacy, defined as >/=5 CMs), PK or PD interactions, potential CM adverse effects and CM adherence. Factors associated with CM polypharmacy and with imperfect adherence were identified using multivariable logistic regression. RESULTS: Of 522 participants, 392 (75%) took a CM (mostly cardiovascular, non-prescription or antidepressant). Overall, 280 participants (54%) had polypharmacy of CMs and/or a drug interaction or contraindication. Polypharmacy was present in 122 (23%) and independently associated with: clinical trial participation, renal impairment, major comorbidity, hospital/general practice-based HIV care (vs. sexual health clinic), and benzodiazepine use. Seventeen participants (3%) took >/=1 CM contraindicated with their ART; and 237 (45%) had at least one PK/PD interaction. CM use was significantly associated with sleep disturbance and myalgia; and polypharmacy of CMs with diarrhoea, fatigue, myalgia and peripheral neuropathy. Sixty participants (12%) reported imperfect CM adherence, independently associated with: requiring financial support, foregoing necessities for financial reasons, good/very good self-reported general health, and >/=1 bed day for illness in the previous 12 months. CONCLUSIONS: In a resource-rich setting with universal healthcare access, the majority of this sample took a CM. Over half had at least one of CM polypharmacy, PK, or PD interaction. Concomitant medication use was associated with several adverse clinical outcomes.
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