Generic drugs can be as much as 90 percent cheaper than name-brand prescriptions. And more than 70 percent of prescription medications taken today are generics. They are a tremendous bargain, and the vast majority are absolutely safe and effective. But they're not necessarily identical to name-brand drugs, and that's what causes confusion and concern.
When Sassee Ann Arnold had a bad reaction to a generic medication; the physical pain was terrible but not being able to do things she loved hurt more.
"I was devastated," she told "Good Morning America."
Sassee has an underactive thyroid, but she's managed it easily for 35 years with a prescription medicine called Synthroid. Then, a few months ago, she received the generic version, levothyroxine, instead.
"It actually hurt to pick my grandchildren, my youngest up. It actually hurt," she said. "I hated climbing the stairs to go to bed at night. The pain would shoot down to my left leg to my feet. I would get charley horses in my toes and feet. My hair started falling out."
When Sassee switched back to the familiar name-brand drug, her painful symptoms vanished.
"I'm ambitious. I've got back in the woodshop, sewing, taking care of the grandchildren," she said.
There are hundreds of anecdotes like Sassee's posted on peoplespharmacy.com.
"GMA" asked the Food and Drug Administration if generic drugs are as safe and effective as their name-brand counterparts.
"Generic drugs are 70 percent of all prescriptions filled, So a huge number of Americans -- the majority of Americans taking medicine -- are well maintained on their generic drugs, and they're satisfied with those drugs," said Dr. Janet Woodcock of the Food and Drug Administration.
But the FDA acknowledged that a handful of prescription medications, including the one Sassee takes, have what's called a "narrow therapeutic window," meaning that too little isn't effective and too much is toxic, and switching can be difficult.
"For the very sensitive drugs, we do look more carefully at the generic applications. We look at the chemistry and the manufacturing and the bioequivalance studies very carefully to make sure they're going to meet our standards," Woodcock said.
Here's how it works: Generic medications are required to contain the exact same amount of active ingredient as the original, but the filler ingredients that bind the medicine together can be different. Some patients have negative reactions to these filler ingredients.
To be fair, others, such as 9-year-old Christopher Holloway who takes an epilepsy drug, do better with the filler ingredients in the generic than they do with the name brand.
"We've had seizure control for the past six years on that generic," Chistopher's mother, Toni Holloway, told "GMA."
Generic drugmakers are required to conduct tests to prove that the amount of active ingredient absorbed into the bloodstream from their copies is equivalent to the originals. The FDA said the average variance between the name brand and the generic is 3.5 percent, but about 50 medications vary by 10 percent or more.
"We feel in those cases that that difference won't have clinical impact on the patient," Woodcock said.
Some medications, such as the sleep aid Ambien, have more than a dozen different generic versions. And pharmacies sometimes switch among them to get the best bargains.
"So this month you got the one that was manufacturer X, the next month it's manufacturer Y, the month after that you got manufacture Z," said Dr. Rana Said, a pediatric neurologist. "So it's the switching that really causes all the problems."
Another wrinkle: About half of generics are made by the company that also makes the name brand of the drug. They're called "authorized generics," and they're exactlyt the same, except maybe they're a different color.
These days, Sassee Ann Arnold asks her doctor to write "do not substitute" on her prescriptions. If you want to get a specific brand -- or generi c-- medication, you should do the same.
While there is no official list, the following drugs are generally accepted to have a narrow therapeutic index.
Thyroid medications: Brand-name: Levothroid, Levoxyl or Synthroid Generic: Levothyroxine
Blood thinners: Brand-name: Coumadin Generic: Warfarin
Heart medications: Brand-name: Digitek, Lanoxin Generic: Digoxin
Psychiatric drugs: Brand-name: Eskalith, lithobid Generic: Lithium
Antiseizure Medications: Brand-name: Dylautin Generic: Phenytoin
Brand-name: Tegretol Generic: Carbamazepine