Nano brows, also known as nano needling or microblading, is a semi-permanent tattooing technique that creates hair-like strokes to fill in sparse or thinning brows. It's a popular beauty treatment that can enhance the shape and fullness of your brows, but like any cosmetic procedure, it has its downsides.

1. **Risk of Infection:** Nano brows involve the use of tiny needles to deposit pigment into the skin. As with any procedure that breaks the skin, there's a risk of infection if the area isn't properly cleaned and cared for post-procedure.

2. **Allergic Reactions:** Some people may have an allergic reaction to the pigments used in nano brow treatments. This can result in swelling, redness, or itchiness around the treated area.

3. **Pain and Discomfort:** While most professionals use a topical anesthetic to minimize discomfort during the procedure, some people still experience pain or discomfort both during and after the treatment.

4. **Semi-Permanent:** While some people may see this as a pro, others might not. The results of nano brows typically last between one to three years, depending on your skin type and lifestyle. This means you'll need to have the procedure redone if you want to maintain the look.

5. **Cost:** Nano brow treatments can be quite costly, especially considering they need to be touched up every one to three years. The cost can vary widely depending on the professional and location, but it's generally a significant investment.

6. **Inconsistent Results:** The results of nano brows can vary greatly from person to person, and even from one treatment to the next. This is due to differences in skin type, lifestyle, and the skill of the professional doing the treatment.

Before deciding to get nano brows, it's important to do your research, understand the potential risks, and consult with a reputable professional.