What do black strips inside the big toenails on both feet indicate?
I have gone through your complaints and reviewed the attached photographs.
It seems to be longitudinal melanonychia.
It has multiple causes the most common being racial (common in dark-skinned individuals), hereditary, certain drugs, subungal naevi and cyst and rarely carcinoma and malignant melanoma.
Since it is present on both the feet it seems to be benign in nature and nothing to worry about.
Still, you can consult a dermatologist and go for dermoscopic examination of the nails.
Hope I have answered your query.
Dr Asmeet Kaur Sawhney, Dermatologist