What you are describing is known as tenesmus. It is defined as constant feeling of the need to empty the bowel, accompanied by pain, cramping, and involuntary straining efforts.
Causes include the following:
Inflammatory bowel disease (IBD)
Infectious colitis (infection of the colon)
Colorectal cancer or tumors
Radiation proctitis or colitis (inflammation of the colon or rectum from radiation)
Perhaps if the thought was infections colitis, then antibiotics would help. Tenesmus can also be caused by prostatitis - in which case antibiotics would help. However, without a clear diagnosis I cannot be sure what is causing your symptoms.
In any case, I agree with you - some sort of endoscopy should be performed, either a colonoscopy or flexible sigmoidoscopy, to evaluate for the causes I mentioned. You may want to obtain a second opinion for further consideration of your symptoms.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Jerry, I don't want to scare you, but this feeling of having to "go" when you do not really have to, can sometimes be a sign of colon cancer. However, it could be a symptom of something not so serious. I am frankly quite shocked that a gastro doctor would prescribe an antibiotic for something like this and not suggest a sigmoidoscopy or colonoscopy. A sig can only visualize 1/3 of the colon. A colonoscopy is the gold standard and can examine the entire colon. If something suspicious is seen during a sig, then they will want to do a colonoscopy anyway and then you have to go thru the prep again. I would stronly urge you to ask for a colonoscopy, and if your current gastro doctor does not go along with it, then find another one. This test can save your life. Don't wait. Again, I am sorry if I am scaring you, but better safe than sorry.
Hemorrhoids could be the cause, even though they weren't seen: they may not bulge when you are lying down. However, it's also true that a definite answer needs to be found. It doesn't seem likely to be a prostate infection if your prostate wasn't enlarged and tender on exam. A flexible sigmoidoscopy, which usually doesn't require the full prep of a colonoscopy, and which can be done in the office, will visualize all of the colon that could be giving such a symptom. And if it shows nothing, it might be a good idea to have a CAT scan of abdomen and pelvis to be sure there's not something outside the colon/rectum causing pressure. Sometimes it's useful to do an exam with an anoscope and finger after the person has squatted and strained a bit, to look for hemorrhoids or partial prolapsing of the rectum.
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