Background: Fumaria officinalis is largely used in traditional medicine due to its efficiency in the treatment and prevention of numerous diseases and its large spectrum of therapeutic effects. Its multiple beneficial properties are due to its richness in bioactive substances, particularly isoquinoline alkaloids. However, few studies have addressed the toxicity of this plant. Objectives: The present work aimed to study acute and subacute toxicity of alkaloids extracted from F. officinalis using Swiss albino mice as the in vivo model. Methods: Alkaloids from the aerial parts of F. officinalis were extracted and administered to male and female Swiss albino mice. The acute and subacute toxicities were studied by monitoring the weight and histopathological study of animal bodies and organs (e.g., liver, heart, spleen, and kidneys). Results: The results revealed that mice treated with increasing doses developed serious symptoms of toxicity (i.e., respiratory problems, tremors, coma, and paralysis leading the death) and lost weight. The LD50 was estimated at 1341.11 mg/kg permitting its classification as a low-toxic plant. The microscopic observations demonstrated disturbances in the kidney and liver, but not the heart and spleen. Conclusion: The alkaloids of the aerial parts of F. officinalis expressed severe toxicity in mice, particularly at high doses. Nevertheless, the neutral fraction of alkaloids is more indicated.