When considering options to prevent pregnancy without sacrificing intercourse by abstaining, both men and women have a multitude of options. For men, the most commonly mentioned way to prevent conception would be the condom.
While not always an accepted method of contraception in various territories (usually because of ridiculous cultural norms or even more ridiculous religions ones), it is among the most widely used in the world. For women, the most commonly used method would likely be birth control pills, which usually encounter the same religion-fueled objections that condoms do, but are less likely to earn a culture-inspired dislike. However, medically speaking, there might be some problems for women who regularly use birth control pills.
Birth control pills do exactly what the name implies, which is control certain aspects of the female physiology to prevent conception. For the most part, these pills are not dangerous and pose no side effects that can be seriously harmful, even if they aren't always 100% effective. However, given the unpredictable and varying nature of human bodies and the unique biochemistry of every individual human being, there are bound to be problems at one point or another. Also, not all birth control pills work on the same base formula, even if they have the same effects. These different formulas may react to certain physiologies or blood chemistry states differently, which can lead to potentially harmful and unexpected results.
Irregular menstruation is frequently cited as one of the possible side effects of birth control pills. This is likely because some of these pills work by altering the flow of hormones in the female body. While these alterations normally should not have an effect on a woman's menstrual cycle, some more sensitive bodies (or some more potent formulas) can disrupt the natural flow of the body. Usually, this is accompanied by swelling and sensitivity in the breasts, almost as if the subject was pregnant, as some observers have noted. Again, this is traceable to the effects some birth control pills have on the delicate hormonal balance of the female body.
Some types of birth control pills are also kept away from people who have or have had certain medical conditions. Some forms of birth control are not considered safe for people who have had liver tumors or liver disease, heart disease, diabetes, breast cancer, reproductive cancer, blood clotting, and high blood pressure. Various side effects can arise from prescribing certain types of pills to people who have had these problems, such as nausea, vomiting, constipation, mood swings, depression, anxiety, and a loss of bone density. As with irregular menstruation, these have been traced back to hormonal imbalances, mainly from a sudden change in the levels of progesterone and oestrogen (not estrogen, which is a completely different hormone) in the body. It has not yet been determined why certain illnesses, combined with changes in hormone levels, bring about the above effects.
It should be noted that the above cases are considered to be rare, and birth control pills should be safe for most women to use as a contraceptive. As with any other medication, if anyone experiences sudden or unexpected effects after taking them, then a doctor should be consulted. If necessary, then different medication may be prescribed to avoid further side effects or damage.