A Research Study to See How a Weekly Insulin, Insulin Icodec, Helps in Reducing the Blood Sugar Compared to Daily Insulin Glargine, Both in Combination With Insulin Aspart, in Adults With Type 1 Diabetes

• Insulin icodec (Drug): Insulin icodec will be administered as subcutaneous injection
• Insulin glargine (Drug): Insulin glargine will be administered as subcutaneous injection.
• Insulin aspart (Drug): Insulin aspart will be administered as a subcutaneous injection.

Primary outcomes

• Change in glycosylated haemoglobin (HbA1c) [From baseline (week 0) to week 26]: Measured in percentage (%)-points.

Secondary outcomes

• Change in time in range 3.9-10.0 millimoles per liter (mmol/L) (70-180 milligrams per deciliter (mg/dL)) [From baseline (week-2-0) to week 22-26]: Measured in %-points.
• Time spent less than (<)3.0 mmol/L (54 mg/dL) [From week 22 to week 26]: Measured in % of time.
• Change in time spent greater than (>)10.0 mmol/L (180 mg/dL) [From baseline (week-2-0) to week 22-26]: Measured in %-points.
• Number of severe hypoglycaemic episodes (level 3) [From baseline (week 0) to week 31]: Measured as number of episodes.
• Number of clinically significant hypoglycaemicepisodes (level 2) (<3.0mmol/L [54 mg/dL],confirmed by blood glucose (BG) meter) [From baseline (week 0) to week 31]: Measured as number of episodes.
• Number of clinically significant hypoglycaemic episodes (level 2) (< 3.0 mmol/L [54 mg/dL], severe hypoglycaemic confirmed by BG meter) or episodes (level 3) [From baseline (week 0) to week 31]: Measured as number of episodes.
• Number of continuous glucose continuous glucose (CGM)-based clinically significant hypoglycaemic episodes (level 2) (< 3.0 mmol/L [54 mg/dL]) [From baseline (week 0) to week 31]: Measured as number of episodes.
• Mean total weekly insulin dose [From week 24 to week 26]: Measured in insulin units.
• Change in body weight [From baseline (week 0) to week 26]: Measured in Kilogram (kg).

Sex: All, ages: 18 Years to any

Inclusion Criteria:

* Diagnosed with type 1 diabetes mellitus greater than or equal to (≥) 1 year before screening.
* Treated with multiple daily insulin injections (daily basal insulin analogue and bolus insulin analogue regimen) ≥ 6 months before screening.
* HbA1c from 7.0-10.0 percentage (%) (53.0-85.8 millimoles per mole (mmol/mol)), both inclusive, at screening confirmed by central laboratory analysis.
* Ability and willingness to adhere to the protocol including performance of self-measured plasma glucose (SMPG) profiles, based on the investigator's judgement.

Exclusion Criteria:

* Known or suspected hypersensitivity to study intervention(s) or related products.
* Previous participation in this study. Participation is defined as signed informed consent.
* Female who is pregnant, breast-feeding or intends to become pregnant or is of childbearing potential and not using adequate contraceptive method.
* Exposure to an investigational medicinal product within 90 days or 5 half-lives of the investigational medicinal product (if known), whichever is longer, before screening.
* Any condition, except for conditions associated with type 1 diabetes mellitus, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol.
* Anticipated initiation or anticipated change in concomitant medications (for more than 15 consecutive days) known to affect weight or glucose metabolism (e.g., treatment with thyroid hormones, or systemic corticosteroids).
* Known hypoglycaemic unawareness as indicated by the Investigator according to Clarke's questionnaire question.
* Recurrent severe hypoglycaemic episodes within the last year as judged by the investigator.

82 more sites not listed.

• Clinical Transparency (dept. 2834), Study director, Novo Nordisk A/S