Aminosalicylates are the most frequently prescribed treatment for ulcerative colitis (UC).

In the absence of empirical evidence, clinicians are uncertain whether to continue aminosalicylates in patients with UC after escalating therapy.

To quantify concomitant aminosalicylate use in UC randomised clinical trials (RCTs), identify factors associated with their use, and estimate treatment costs of concomitant aminosalicylate therapy.

MEDLINE, Embase, and CENTRAL were searched from inception to 1 March 2017 for placebo‐controlled RCTs of immunosuppressants, biologics, or oral small molecules in adults with UC.

The proportion of patients prescribed concomitant aminosalicylates at trial entry was pooled using a random‐effects model. Meta‐regression was performed to assess trial‐level factors associated with aminosalicylate use. Treatment costs were estimated using 2018 formulary data from five Canadian provinces.