Until recently, Section 17a (2) of the North Rhine-Westphalian forensic mental health law regulated the compulsory treatment of patients who lack mental capacity with the goal to “achieve the ability to be discharged”. Little has been discovered so far about the effects of such compulsory treatment. The present study evaluates the effects of compulsory antipsychotic medication in patients with a schizophrenia spectrum disorder who are detained in German forensic hospitals. Therefore, the symptom severity and level of functioning were assessed by mental health professionals directly before the first application of all compulsory medications (n = 16) which were administered in 3 forensic psychiatric hospitals of the Regional Association of Westphalia Lippe in 2019, as well as 4 weeks and 6 months afterwards using Positive and Negative Syndrome Scale (PANSS) and Global Assessment of Functioning (GAF). The achievement of other treatment goals, the assessment of the ability to be discharged and an overall evaluation of the compulsory treatment were also questioned. There was a significant reduction of the PANSS factors “positive symptoms” and “excitable/hostile” and an increase of the level of functioning at the first catamnesis time point, both of which continued slightly weakened. Only two patients showed no or a negative effect. Although the effects on the questioned treatment goals fell short of expectations, the mental health professionals predominantly assessed the compulsory medication positively. Compulsory treatment with antipsychotics can have a favorable effect on the goal of “achieving the ability to be discharged”; however, the limited effect on the insight into the illness and the necessity of taking the medication reveal limitations indicating that this measure is not sufficient to achieve the ability to be discharged.