Oxalic acid is a less often used remedy good for headaches, primarily, and pains in general. If you observe that the pain is felt in spots, wherever that might be, think of this remedy.
This remedy has a peculiar symptom. When the sufferer starts to think of their illness everything gets markedly worse. Only use this as a confirming symptom however.
All pains are felt in precise spots
Diarrhea
Headache