(CBS) -- About 80 percent of prescriptions filled in the U.S. are for generic drugs. But as CBS 2's Mary Kay Kleist reports, generics may not always be the best choice.
Debbie LaDeur takes thyroid medication every day. Recently she opted to try a generic version of Synthroid.
"I was led to believe they were the same maybe because of the one same active ingredient," said LaDeur.
But she soon noticed something was wrong.
"I felt more tired. I just didn't feel like myself. Mainly the energy," LaDeur says.
Jan Engle, a doctor of pharmacy at University of Illinois Chicago, says small changes in drug concentration can cause adverse reactions or change the way a drug works.
Besides thyroid medicine like LaDeur's, examples of NTI drugs that might cause problems as generics include blood thinners, anti-seizure drugs and some antibiotics.
Another problem area seems to be with extended-release generics.
"It may have a different release time, and it may peak at a different time, meaning you get the maximum drug effect at a different time than the brand," Engle says.
That's what happened in the case of a popular anti-depressant.
Dr. Tod Cooperman company ConsumerLab tested the Budeprion generic version of Wellbutrin XL after patients started complaining of side effects.
"Instead of being well-controlled in terms of their depression, some were becoming suicidal," he says.
His tests found that Budeprion was not equivalent to the brand.
"The generic was releasing its drug much faster early on, but later on there was very little coming out of that pill," Cooperman says.
Experts say the generic pill is the same medication, but the mechanism that delivers it to your body may be different.
"The extended-release part may be under a different patent than the active ingredient part of the drug," Engle says.
If you notice something's not right, report it to your doctor or pharmacist.
"I recommend starting a diary. Keep track of your symptoms. Keep track of what you think isn't working, or what the side effects are," Engle says.
The FDA says the generic Wellbutrin XL tested by ConsumerLab was pulled from the market just one month ago, after five years of complaints.
Both experts say most generics are safe and work similar to their brand name equivalent.
If you find one that's working well for you, make sure you ask for the same generic manufacturer when you refill your prescription.
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