Trial design, settings and locations of data collection This was a randomized, parallel, placebo-controlled, triple-mask clinical trial, in which the patient, operator and evaluator were masked to the group assignment. A third researcher, not involved in the evaluation process, was responsible for the randomization process, and delivery and guidance on the administration of the drugs. This study was performed from 05/11/2017 up to 01/12/2018 in the city of Cascavel (Paraná, Brazil). All bleaching procedures were carried out within the Clinics of the Dental School of the State University of Oeste do Paraná (Cascavel, Paraná, Brazil).

Sample size calculation The primary outcome of this study was the absolute risk of TS. The absolute risk of TS was reported to be approximately 87% (Tay et al. 2009, De Paula et al. 2013) for the bleaching product Whiteness HP Maxx (FGM, Prod. Odont. Ltda, Joinville, SC, Brazil). Thus, a minimum sample size of 114 patients was required to have a 90% chance of detecting, as significant at the two-sided 5% level, a decrease in the primary outcome measure from 87% in the control group to 60% in the experimental group (which represent a difference of 31% in the absolute risk of TS).

Random sequence generation and allocation concealment. The investigator used blocked randomization (block sizes of 2 and 4) with an equal allocation ratio. The randomization process was performed by software freely available on the internet (www.sealedenvelope.com). Opaque and sealed envelopes containing the identification of the groups were prepared by third party, not involved in the study intervention. This third researcher, not involved in the evaluation process, was responsible for the randomization process, delivery and guidance on the administration of the drugs. A single random sequence was performed for both centers.

Study intervention Patients were divided into acetaminophen/ketorolac tromethamine and placebo groups. All patients received the same bleaching treatment, which was performed by four operators in the Dental School. One hour before in-office bleaching patients received either the acetaminophen 750 mg (Paracetamol 750 mg, Bioativa compounding pharmacy, Cascavel, PR, Brasil) and ketorolac tromethamine oral 10 mg (Toragesic® 10 mg, EMS Sigma Farma, Hortolândia, SP, Brasil) or placebo, for both medicines in identical tablets or capsules. The operator administered the first dose of drug 1 h before the protocol, and extra doses were administered every 8 h for 48 h to keep a safe maximum daily dosage of 4000 mg of acetaminophen and 40 mg of ketorolac tromethamine.

The tablets of Toragesic® and Paracetamol were removed them from their original packaging and inserted them whole into new vials made especially for this research. the investigator stored the capsules in individual vials containing 6 capsules of Paracetamol and 6 capsules of Toragesic® required for each bleaching session. The placebo tablets for Paracetamol contained starch lactose free (Bioativa compounding pharmacy, Cascavel, PR, Brasil) and the placebo capsules for Toragesic® contained base past, lactose free (Oro-tab®, Bioativa compounding pharmacy, Cascavel, PR, Brasil) and One hour before starting the bleaching application, the masked researcher responsible for drug administration gave the first dose to the patient. Then, the investigators isolated the gingival tissue of the teeth to be bleached using a light-cured resin dam (Top Dam, FGM Dental Products), and each tooth was light-cured for 10 s (Radii-cal, SDI, Victoria, Australia). After placement of a lip retractor (Arcflex, FGM), the researcher used the 35% hydrogen peroxide gel (Whiteness HP Automixx, FGM) in a single 50-minute application for both groups in accordance with the manufacturer's directions. Two bleaching sessions were performed 1 week apart. All participants were instructed to brush their teeth regularly using fluoridated toothpaste.