Here is what I found, hope this helps...

iodine contrast injection:

What to expect
Before an Iodine contrast injection, you will be asked several questions about your medical history and allergies. You will be given information about the type of contrast you are going to receive and asked to sign a consent form.

The nurse, technologist or radiologist will administer the injection and monitor you for any reactions.

If your exam requires barium, you will be asked to drink it prior to your scan.

Patient preparation
If your scan requires injection of intravenous contrast, you will be instructed to refrain from eating or drinking for at least two hours before your scan. You may be given more specific instructions for certain exams.

Please let the staff know at the time of scheduling if you are diabetic or are being treated for kidney disease. You will be given specific instructions before your scan.

Side effects and complications
Complications associated with intravenous contrast media are uncommon. Minor side effects such as flushing, a mild rash or nausea sometimes occur. Allergic-type reactions such as severe hives, facial swelling, low blood pressure or other reactions also are possible. Our staff is prepared to administer emergency treatment for these infrequent occurrences.

As with any intravenous injection or blood test, the contrast medium has a slight chance of escaping from the vein and leaking into surrounding tissues. Such leakage, which is called extravasation, may cause bruising and mild pain. Our staff will administer treatment specific for the amount and type of fluid that escaped. Another complication of any intravenous injection is the slight possibility of damage to a nerve.

Barium contrast does not interact with the body. It is an inert substance that passes harmlessly through the gastrointestinal tract. Occasionally it may cause constipation, which can be prevented or alleviated by drinking several glasses of water after your scan.

Follow-up care
Except for the uncommon complications discussed above, no follow-up care is required after receiving contrast medium for a CT scan.


http://www.radiological.com/imgComputedTomography/oralAndIVContrast.cfm?secti...

Injecting anything directly into the blood stream would be far more reactive. Not one assimilation/defense system has much of chance at metabolizing the substance or turning it into something the body can use. They also use other contrasting dye chemicals in the injection cocktail, so you would need to know whether you had a bad reaction to that and be able to exclude it. But even if it was the iodine, you would not likely get the same experience with oral ingestion unless you took high doses over the long-term. My opinion only.