I stepped on a toothpick and i can feel it under my skin but mri couldn't see it. it is painful what should i do?

Salt Water: You could try soaking the foot in a bowl of warm salt water. You can make the salt water yourself, adding enough salt to make it salty but still dissolved. The salt content of the water will gently draw the foreign body out of the foot. Understand that this may not happen right away but should cause movement in the right direction. For pain relief you can try motrin or naprosyn (naproxen) as directed.

Podiatrist/surgeon: Nonmetallic objects are hard to see with many types of imaging studies. If you feel a foreign body, but it's not visible, use crutches. Heat will help it work its way closer to the surface, but that takes time. Without being able to see the object, some docs might use ultrasound to localize it. Care must be taken that removing it won't damage important tissues, nerves, tendons. Ask MD.for referral

Wood Foreign Body?: Usually these things show up on MRI as it can detect wood, which doesn't show up on plain X-ray films. if the object did not show up on MRI, then it has likely been removed completely and there may only be a few small remnant splinters. I assume you had the toothpick removed. Talk with the doc who ordered the MRI. You may have injured nerves in your foot, which is very painful. Followup w/ MD asap