Background Guidelines recommend stepping-down asthma treatment to the minimum effective dose to achieve symptom control, prevent adverse side-effects, and reduce costs. Limited data exist on asthma prescription patterns in a real-world setting. We aimed to evaluate the appropriateness of doses prescribed to a UK general asthma population and assess if stepping-down medication increased exacerbations or reliever use, as well as its impact on costs. Methods and findings We used nationwide UK primary care medical records, 2001-2017, to identify 508,459 adult asthma patients managed with preventer medication. Prescriptions of higher-level medication: medium/high-dose inhaled corticosteroids (ICS) or ICS + add-on medication (long-acting beta-agonist, LABA, leukotriene receptor antagonist, theophylline or long-acting muscarinic antagonist) steadily increased over time (2001=49.8%, 2017=68.3%). Of those prescribed their first preventer, one third were prescribed a higher-level medication of whom half had no reliever prescription, or exacerbation, in the year prior. Of patients first prescribed ICS + 1 add-on, 70.4% remained on the same medication during a mean follow-up of 6.6 years. Of those prescribed medium/high-dose ICS as their first preventer, 13.0% already had documented diabetes, cataracts, glaucoma or osteopenia/osteoporosis. A cohort of 125,341 patients were drawn to assess the impact of stepping-down medication; mean age 50.4 years, 39.4% males, 39,881 stepped-down. Exposed patients were stepped-down by dropping their LABA, or another add-on, or by halving their ICS dose (halving their mean-daily dose or their inhaler dose). The primary and secondary outcomes were, respectively, exacerbations and an increase in reliever prescriptions. Multivariable regression was used to assess outcomes and determine the prognostic factors for initiating step-down. There was no increased exacerbation risk for each possible medication step-down (adjusted hazard ratio, 95% CI, p-value: ICS inhaler dose=0.86, 0.77-0.93, p <0.001; ICS mean-daily=0.80, 0.74-0.87, p<0.001; LABA=1.01, 0.92-1.11, p=0.87, other add-on=1.00, 0.91-1.09, p=0.79), and no increase in reliever prescriptions (adjusted odds ratio, 95% CI, p-value: ICS inhaler dose=0.99, 0.98-1.00, p=0.59; ICS mean-daily=0.78, 0.76-0.79, p<0.001; LABA=0.83, 0.82-0.85, p<0.001; other add-on=0.86, 0.85-0.87, p<0.001). Prognostic factors to initiate step down included medication burden, but not medication side-effects. National Health Service indicative prices were used for cost estimates. Stepping-down medication, LABA or ICS, could save annually around £17,000,000 or £8,600,000, respectively. Study limitations include the possibility that prescribed medication may not have been dispensed or adhered to and the reason for step-down was not documented. Conclusion In this UK study, we observed that asthma patients were increasingly prescribed higher levels of treatment, often without clear clinical indication for such high doses. Stepping-down medication did not adversely affect outcomes and was associated with substantial cost savings.