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There are 2 or 3 differentials of such a rash.
Sweat rash can be a cause. Use talcum powder between your thighs on hot or humid days. Obese individuals are more inclined to develop rash due to increased friction along the folds of skin. Reducing weight can alleviate rash along with several other health benefits.
Wearing proper fitting undergarments can also stop this, (for males) when your boxers or other undergarments, or even pants and shorts dont fit right the stitching of the inside crotch seam rub and also cause rash.
Allergic rash, you can use an anti histamine for a couple of days both locally and orally and see if the rash goes away. Recommended anti histamines that do not sedate are Cetrizine or Loratidine twice daily dose for relief. Both are available over the counter in oral as well as topical forms.
Try to wear well fitted clothes rather than light fabrics when going on a walk or playing a sport.
Jock itch can cause inner thigh rash. The rash is caused by a fungus, T rubrum, which starts at the intersection of the groin and thigh usually on both sides. The rash can advance further down the thigh if not treated properly.
The rash can appear to be scaly with redness near the fold of the groin. Away from the groin it can be reddish-brown and less or not scaly at all.
Wear loose fitting clothing made of cotton.
Do not sharing clothing, towels or washcloths.
Dry the groin completely after showering.
powders or sprays once a day.
--Hope this helps--