Would it negatively affect my health or its effectiveness if i were to be tattooed over the nexplanon implant?

May be: Tattooing is a form of self-mutilation and i would advise strongly against getting one. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.

It's your choice: It won't interfere with the medicine. I'm a tattoo buff and have three. But you need to be VERY sure it's what you want. Wait several years after you get the original idea and if it is EXACTLY what you want, go for it. It hurts. But mine are so meaningful I would not trade the pain for any pleasure I can imagine. Follow your dream, think for a LONG time, and then make the right choice.