This is called heat urticaria.
You may take Benadryl. This is available without the prescription.
This is also called cholinergic urticaria. Any stimulus that causes sweating, including elevated environmental temperature, hot food, sauna baths, immersion in hot water, spicy food, chilies, emotional stress, etc may cause it. Please read about it;
The diagnostic test for it is an intradermal injection of either 0.05 mL of 0.002% carbamylcholine chloride (carbachol) or 0.05 mL of 0.02% (0.01 mg) methacholine which produce a flare-up of cholinergic urticaria. Treatment options for cholinergic urticaria are;
1) antihistamines (cetirizine)/benadryl
2) leukotriene inhibitors (ketotifen), and
Danazol and Beta-blockers, such as propranolol, have also been reported to be useful in treating the disease.
Desensitization with autologous sweat has been reported with good results in patients resistant to conventional therapy who have sweat hypersensitivity. In this, your own sweat is injected in the skin. Consulting with an allergy specialist would be advised.
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