While the eligibility criteria can vary by country and organization, you can’t donate blood if you are under 16, weigh less than 150 lbs (50 kg), are pregnant, or have traveled to areas with a high risk of outbreaks recently. People with some infectious diseases, such as hepatitis B or C, or cancer, are ineligible (12).  Also, if you've had piercings, tattoos, surgery, vaccines, or certain medications like blood thinners or antibiotics recently, you'll need to wait for a period of time before donating blood (3, 4).