SAN ANTONIO -- Tim Tilt has been living with epilepsy for 32 years.

For most of that time, he's been taking the same anti-seizure medicine until a few months ago, when he received a medication that he didn't recognize.

"One day I went to get it and tore the bag open and there were all these different bottles than I was used to," Tilt said.

Tilt received a drug similar to the one he was taking, but not the exact one, so he called his doctor to see if it was safe to take. Tilt's doctor told him to take it, but to monitor any adverse reactions.

"I was itching all over, dead tired and nauseated," Tilt said.

Tilt's doctor quickly switched him back to his original medicine.

Dr. Paul Garcia of WellMed said that the practice of "therapeutic switching" is very common and can be very dangerous.

"The drugs are the same as far as active ingredients, but the drugs can have inactive ingredients that can change absorption," Garcia said.

The speed at which a medicine is absorbed into the body can be the difference between life and death in certain patients, particularly those on medications for cholesterol, antidepressant and anti-seizure medications, Garcia said.

"Say this drug gets absorbed in six hours, and then they take another that gets absorbed in seven hours, in certain drugs that might make a difference in being toxic," Garcia said.

A rule of thumb to use is when your doctor prescribes a medication, always look closely at the name on the prescription and make sure it matches the name on the bottle, Garcia said.