Introduction: Tofacitinib is an oral Janus kinase inhibitor for the treatment of rheumatoid arthritis (RA) and psoriatic arthritis (PsA). This post hoc analysis assessed frequency or duration of early select non-serious adverse events (AEs; excluding infections), and their impact on treatment discontinuation, in patients with RA or PsA treated with tofacitinib 5 or 10 mg twice daily, or placebo. Methods: Data were pooled from five phase 3 and one phase 3b/4 studies in patients with moderate-to-severe RA, and two phase 3 studies in patients with active PsA. Select all-causality, non-serious AEs, reported to month 3 (placebo-controlled period), were headache, diarrhea, nausea, vomiting, and gastric discomfort (including dyspepsia, gastritis, epigastric discomfort, and abdominal discomfort or pain); incidence rates (unique patients with events per 100 patient-years of follow-up), duration of, and discontinuations due to these non-serious AEs were reported. Results: We analyzed 3871 and 710 patients with RA and PsA, respectively. Incidence of non-serious AEs to month 3 was generally similar with tofacitinib and placebo. The most frequent non-serious AEs were headache and diarrhea with tofacitinib, and dyspepsia, nausea, and headache with placebo. Most events were mild or moderate in severity, lasting ≤ 4 weeks. Permanent discontinuations due to non-serious AEs were not observed in patients with PsA, and were < 1.0% in patients with RA across treatment groups. The most frequent cause of temporary discontinuation across all groups was gastric discomfort (0.3–0.8%). Conclusions: Non-serious AE incidence was generally similar in patients with RA or PsA receiving tofacitinib or placebo. Most events were mild or moderate and generally resolved within 4 weeks. Trial Registration: ClinicalTrials.gov identifiers: NCT00960440; NCT00847613; NCT00814307; NCT00856544; NCT00853385; NCT01877668; NCT01882439; NCT02187055. Tofacitinib is a medicine that can be taken by patients to treat rheumatoid arthritis (RA) or psoriatic arthritis (PsA). Serious side effects that might occur in patients taking tofacitinib are more frequently discussed than the mild, non-serious side effects that patients might consider to be more of a ‘nuisance’, which often occur shortly (< 3 months) after starting treatment. Here we looked at patients with RA or PsA who were taking tofacitinib or placebo (no medicine) during clinical trials, to find out how often they had certain non-serious side effects, how long they lasted, and whether they caused the patients to stop taking their medication. A similar number of patients with RA or PsA taking tofacitinib or placebo had non-serious side effects. The most common non-serious side effects in patients taking tofacitinib were a headache and diarrhea. The most common non-serious side effects in patients taking placebo (no medicine) were indigestion, a feeling of sickness, and/or headache. Most non-serious side effects were mild or moderate and stopped within about 4 weeks. Fewer than one in every 100 patients with RA, and no patients with PsA, stopped taking their medication because of non-serious side effects. Most patients who stopped taking their medication did so due to a feeling of gastrointestinal (stomach) discomfort.