Which sentence is better?

Both have problems.
A sentence should always start with a capital letter.
In the first sentence you talk about symptoms (plural), then say "similar to that," which doesn't match the tense; it should be "similar to THOSE"

In the second sentence you use "remedies" instead of medicine. There is no such thing as a "small amount of remedy" Something is remedied, or it isn't.

I would offer this instead:
"A method of treating disease with small amounts of medicines that, in larger dosages, produce in healthy subjects symptoms similar to those being treated."

In the first sentence you talk about symptoms (plural), then say "similar to that," which doesn't match the tense; it should be "similar to THOSE"

In the second sentence you use "remedies" instead of medicine. There is no such thing as a "small amount of remedy" Something is remedied, or it isn't.

Incorrect. Remedy (noun) can be used to mean "medicine or treatment for a disease or injury". It is thus correctly used by oristarA. It is often used in connection with homeopathy e.g. "I use the smallest amount of remedy possible to get the maximum effect in the patient", and also mainstream medical writing e.g. "Insulin and some vaccines are given subcutaneously. The length of the needle used must be between 23 to 25 gauge and the volume of remedy administered is no larger than 1ml."

Remedy: a medicine or therapy that cures disease or relieve pain.