>>The past two feedings my ball python threw up his food. I give him 1 live mouse hopper a week. He is 3 months old. He has gotten very skinny but seems to be active. Any ideas?

If it were me I would hold off feeding him for two full weeks. No handling him either. Be sure he has water and hides. CHECK all his temps. Regurg out of the blue can often be a temperature change....especially in the fall with lower outside temps. ALSO, make sure his temps are TOO high. I think many people err on the side of too hot, which is deadly and can also lead to regurging.
Clean his cage while you are checking everything else. Then let him rest and rebuild his stomach acids. Then try something a bit smaller than what he has been taking.
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Sonya

I'm not mean. You're just a sissy.
Happy Bunny