We tried it one time with our son. He was 13 at the time. He's 15 now. We noticed no significant difference. I guess you should do it more than once to notice a difference but since we saw absolutely no change, we didn't pursue it.
I have worked with a few families in the UK that have used secretin. The results have been varied. From a report of vast improvements in behaviour, eyecontact language etc to one family seeing initial changes but then a regression. Through to no noticeable change other than a change in bowel movements. One family did report an increase in undesired behaviours eg aggression and biting. These were all anecdotal reports and not medically supported so the secretin may not have been factor causing the changes. One family noted an effect from the herbal secretin and not the pig produced secretin.
I think that its the same as the special diets, it may have an effect on some people and not on others. If you feel comfortable with its use good luck.
If you have doubts then don't give it.
My son was in an initial study for synthetic secretin in AZ when he was 5 yrs old and non-verbal. After the initial infusion and within six months the speech pathologists recorded an increase in his receptive language of 18 months and 12 months for his expressive language. It was a double blind study and afterwards we we told he received just a half dose. We were excited at the prospect of something actually working. He is now 10 yrs old and the FDA is no closer to approving this drug than they were 5 years ago. He has had no secretin since. The improvements made have not seemed to regress. We considered going out of the U.S. to get the porcine secretin but were told that there was a high chance of there being 'stealth' viruses in the medication, plus no doctor in the U.S. who prescribe the porcine formula for autism in fear of having his license revoked. Currently the doctors who were so heavily in the synthetic secretin research have lost a lot of steam due to the FDA dragging there feet and making it very difficult to get it approved without a lot more expensive studies.