Cancer doesn’t appear overnight. In most cases, it develops silently in the body for months, or even years, before being diagnosed. While not all unusual symptoms directly indicate the presence of cancer, certain changes can be early warning signs of abnormal cell growth. Paying attention to these signs can make all the difference for early diagnosis and treatment.

Here are 12 possible early signs of cancer cells that should never be ignored.

1. Unexplained fatigue

Persistent fatigue that doesn’t improve with rest can be a sign of a more serious problem than simple stress. Cancer cells deplete energy, weaken the immune system, and disrupt nutrient absorption, which can lead to shortness of breath.

2. Sudden and unexplained weight loss

A weight loss of more than 5 to 10% in a short period of time, without dieting or exercise, can be a warning sign. Many cancers, particularly those of the stomach, pancreas, and lungs, cause rapid weight loss.

3. Persistent pain

Chronic pain that persists without an apparent cause should be taken seriously. For example:

Back pain can be a sign of bone or ovarian cancer.

Headaches that do not respond to treatment may be related to brain tumors.