Background: In dual antiplatelet therapy (DAPT), low-dose acetylsalicylic acid is combined with a P2Y12 inhibitor. However, combining antithrombotic agents increases the risk of bleeding. Guidelines on DAPT recommend using this combination for a limited period of between three weeks and 30 months. This implies the risk of DAPT being erroneously continued after the intended stop date. Objective: The primary objective of this study is to assess the proportion of hospitalized patients treated with DAPT whose treatment deviated erroneously and unintentionally from the guidelines. We also assessed risk factors and the effect of a pharmacist intervention. Methods: All patients admitted to the Spaarne Gasthuis (Haarlem/ Hoofddorp, the Netherlands) who used DAPT between March 25th, 2019, and June 14th, 2019, were, in addition to receiving regular care, reviewed to assess whether their therapy was in line with the guidelines’ recommendation and whether deviations were unintended and erroneous. In the event of an unintended deviation, the pharmacist intervened by contacting the prescriber by phone and giving advice to adjust the antithrombotic therapy in line with the guideline. Results: We included 411 patients, of whom 21 patients (5.1%) had a treatment that deviated from the guidelines. For 11 patients (2.7%), the deviation was unintended and erroneous. The major risk factor for erroneous deviation was the use of DAPT before hospital admission (OR 18.7; 95%CI 4.79–72.7). In patients who used DAPT before admission, 18 out of 58 (31.0%) had a deviation from the guidelines of whom 8 (13.8%) were erroneous. For these eight patients, the pharmacist contacted the prescriber, and in these cases the therapy was adjusted in line with the guidelines. Conclusions: Adherence to the guidelines recommending DAPT was high within the hospital. However, patients who used DAPT before hospital admission had a higher risk of erroneous prescription of DAPT. Intervention by a pharmacist increased adherence to guidelines and may reduce the number of preventable bleeding cases.