Answer
Spontaneous erections are a recognised effect. An erection lasting longer than 4 hours is a medical emergency requiring A&E assessment.
Sources for this answer
- IMCIVREE 10 mg/ml solution for injection — Current selected SmPCemc · Summary of Product Characteristics
- NHSNHS · NHS medicine guidance
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.