Both prescribed medication and street drugs can disrupt your cycle and may affect your fertility.
Marijuana is known to disrupt women's cycles not to mention more powerful street drugs! Even tobacco and caffeine can lower your fertility, and anti-histamines can affect sperm survival.
So let your GP know if you are trying to conceive so that your doctor can take it into account when considering what to prescribe. However, this does not mean that any of these things can be considered to be contraceptives!
Of course if you miss a period and you are sexually active, even if you are using contraception, the most likely cause is that you are pregnant.
Make sure you take a pregnancy test as soon as possible to make sure. The sooner you have proper ante-natal care the better for you and the baby.
If there is no chance whatsoever that you are pregnant, go and see your GP for a check-up, especially if you've missed more than one period.
If you want to know more about your periods, pregnancy or other aspects of women's health, take a look around the rest of our website.