In the book Flowers for Algernon, what was Charlie warned of before the operation?

In the book Flowers for Algernon, Charlie Gordon undergoes an experimental surgery in an attempt to improve his low IQ. The surgery has previously been done on a mouse named Algernon, who Charlie meets during the story and develops love and respect for. At the beginning of the story, Charlie's IQ is only 68, and he attends a literacy class for adults with learning disabilities. His teacher, Miss Kinnian, recommends him for the surgery based on his motivation for learning. The scientists who developed the experimental surgery, Dr. Strauss and Professor Nemur, hope that the surgery will triple Charlie's intelligence.

Before the surgery, Charlie is warned that the effects of the procedure may not be permanent. Charlie was willing to undergo the surgery in the hopes that intelligence might make him more likable and bring new friends. After the surgery, Charlie does experience significant improvement in his intelligence, but it does not last.