I'am 18years old.i feel a great pain while putting my foreskin back.as soon as my penis erects the pain also increases.what should i do?

Phimosis

A medical condition whereby the uncircumcised foreskin cannot be pulled back beyond the widest part of the penis head. It's a common complaint for men where the foreskin is excessively long, or if the skin has been torn and healing has lead to the foreskin contracting.

Remedy: If the foreskin is very tight, circumcision is often the order of the day. There is no medical need to get rid of excess foreskin - providing it slides freely over the head and the area exposed can be kept clean.

Paraphimosis

This occurs when a tight, uncircumcised foreskin gets pulled back and stuck behind the head of the penis (or glans). When this happens the glans swells up, which can be deeply painful.

Remedy: Hobble along to your GP or genitourniary medicine (GUM) clinic (call your local hospital switchboard or search the fpa's clinic finder for details). A professional pair of hands should be able to squeeze out the fluid and return the foreskin to its original position. If it's too inflamed, however, then the skin may be slit to allow the foreskin to return back over the head. Once the swelling had subsided, and the incision healed, a circumcision may be advised to prevent it happening again.

Smegma build-up

Properly known as smegma bacillus, this is a naturally occurring white or waxy deposit that is secreted by the penis glands. If left unwashed, it can collect under the warm, moist hood of the foreskin and become a breeding ground for bacteria. Not only can it get really stinky, a few scientific studies suggest there is a link between smegma bacillus and penile cancer.

Remedy: Wash twice a day by gently retracting the foreskin and cleaning with unperfumed soap and water. Be sure to dry the head before putting it back to bed.
Grin.

Thrush

If your foreskin and penis head is red and inflamed, then there's a chance you may have thrush. The condition is caused by a harmless yeast-like fungus which occurs naturally in the vagina and is controlled by the presence of certain bacteria. This bacterial balance can be easily upset, however, by anything from antibiotics to stress or tight-fitting clothes. As a result the fungus multiplies, becomes infectious, and can be passed on through sex.

Remedy: Thrush can be easily treated with over-the-counter creams like Canestan. Applying natural yoghurt can also relieve the pain. Avoid sex until the infection has cleared.

Phimosis. Basically, you can try and stretch it. Maybe you just have to be a bit more careful and use lubrication. If the problem persists, talk to your doctor or an urologist; they will probably recommend circumscision.