What are the side effects of treatment for mastocytosis?

Minimal. Antihistamines are the mainstay of treatment. Older antihistamines may cause sedation and should be used with caution in the elderly, but newer one are generally tolerated well. For systemic mastocytosis with diarrhea, some people use cromolyn. This also has relative few side effects, with a few people experiencing GI upset. Always consult your doctor before taking any new medications.