se faire du mauvais sang

You use it as a reflexive verb. This means you need to match the pronoun to the subject. For example, je me fais or tu te fais. It works perfectly in the present tense for current stress. You can also use it in the past tense. Use je me suis fait du mauvais sang for a finished worry. It is like a physical weight you carry until the problem is solved.

Use it when someone is late coming home. Use it when you are waiting for medical results. It is perfect for talking to close friends about your fears. You can also use it to tell someone else to relax. Tell them: Ne te fais pas de mauvais sang ! This is a common way to say 'Don't worry about it.' It shows you understand their stress is heavy.

Do not use this for trivial, tiny things. If you forgot your umbrella, it is too dramatic. It is not for professional, cold business reports either. Avoid it if you are just 'busy' rather than 'worried.' It implies a deep emotional or physical reaction to stress. Also, do not use it to mean 'taking time.' That is a common mistranslation; it is about anxiety, not duration.

This expression dates back to old medical theories about 'humors.' People used to believe emotions physically changed your blood. 'Bad blood' was thought to cause actual illness. In French culture, expressing your worries is quite common. It shows you care deeply about the person or situation. It is a very human, visceral way to connect with others.

You might hear se faire de la bile which means the same thing. This refers to 'bile' instead of blood. Another one is se ronger les sangs, which is even more intense. That one implies the worry is literally 'gnawing' at you. If you want to be more modern, you might just say stresser. But se faire du mauvais sang remains a classic, soulful choice.