Background and Importance Pain is an unpleasant sensory and emotional experience associated, or similar to that associated, with actual or potential tissue damage. This is a dominant symptom in cancer patients and affects their day-to-day life. The World Health Organization published an analgesia implementation model consisting of three levels. This model includes adjuvant analgesics, which are drugs marketed for indications other than pain, very useful when associated with opioid therapy.

Aim and Objectives The aim of the study was to collect and analyse the drug interactions that exist in the concomitant use of adjuvant analgesics used to control pain in patients with breast cancer undergoing oral therapy, in a hospital institution.

Material and Methods A list of adjuvant analgesics and oral medications used in the treatment of breast cancer was drawn up.

The adjuvant analgesics studied were carbamazepine, gabapentin, oxcarbazepine, pregabalin, amitriptyline, duloxetine, venlafaxine, dexamethasone, methylprednisolone and prednisolone. The oral breast cancer drugs were abemaciclib, capecitabine, everolimus, lapatinib, olaparib, palbociclib, ribociclib, tucatinib and vinorelbine.

Cancer Drugs Interaction, Drugs.com and Lexicomp® were consulted and the interactions were collected, evaluated and divided into four groups: 1) severe interaction, 2) moderate interaction, 3) weak interactions and 4) no known interactions.

Tables were created and analgesic adjuvants interaction rates were calculated.

The following results were obtained 1) Severe interaction: carbamazepine (77,8%), oxcarbazepine (55.6%), dexamethasone (33.3%), amitriptyline (11.1%) and venlafaxine (11.1%).

2) Moderate interaction: dexamethasone (55.6%), methylprednisolone (33.33%), oxcarbazepine (33.33%), prednisolone (33.33%) and venlafaxine (33.33%), amitriptyline (11.1%), carbamazepine (11.1%) and duloxetine (11.1%).

3) Weak interactions: amitriptyline (22.2%) and venlafaxine (22.2%) and methylprednisolone (11.1%) and prednisolone (11.1%).

4) No known interactions: gabapentin (100%), pregabalin (100%), duloxetine (88.9%) amitriptyline

(55.6%), methylprednisolone (55.6%), prednisolone (55.6%) venlafaxine (33.3%), carbamazepine (11.1%), dexamethasone (11.1%) and oxcarbazepine (11.1%).

Conclusion and Relevance The study concludes that there are many serious, moderate and weak interactions to be taken into account when treating pain in patients undergoing oral therapy for breast cancer. Depending on the degree of interaction, the pharmacist may suggest replacing or closely monitoring these patients.

These data reinforce the importance of the pharmacist as an element of the healthcare team, providing information in decision-making process and improving patient therapeutic outcomes.

References and/or Acknowledgements 1. https://cancer-druginteractions.org/checker

2. https://www.drugs.com/drug_interactions.html

3. https://www.uptodate.com/drug-interactions

Conflict of Interest No conflict of interest.