Alcohol:
Any more than a glass of wine will start to make you drowsy, dulling the senses and making you less receptive to stimulation. If a man regularly drinks to excess, it will make him less fertile.

Drugs and some medicines:
Many types of medication on the market at the moment (especially cold and flu remedies) will make you sleepy, so it is always important to read the notes in the packet - for 'do not use while operating heavy machinery or driving', read 'do not use whilst trying to seduce your partner'. Recreational drugs almost always have a negative effect on libido, even if a man feels stimulated, he will find it hard to maintain an erection.

Pesticides:
Many of the chemicals used in farming weaken sperm and are thought to casue male infertility. Always wash your fruit and vegetables and eat organic produce whenever you can.

Viagra:
We all know the physical effect viagra has on men, but they need to feel desire to be turned on, not just a physical reaction to a chemical. Natural aphrodisiacs, which have not necessarily been scientifically proven, are more like mysterious love potions. If you eat a bowl of ginger soup with your lover before a night of hot sex, you are not quite going to know what started it all off. With viagra you know all too well.

Stress:
It may not be a substance, but it has an extremely powerful effect on the libido. It is impossible to really enjoy sex, if you haven't had enough sleep, are running through your work problems at the same time or have one eye constantly on the bedside clock. Make time for pleasure.

Television:
Modern life is hectic, our days are too fulll and our evenings too short. Studies show that if you come home and slump in front of the TV before 8:30pm, you are more likely to fall asleep in front of the small screen. Try lighting candles and working on the atmosphere instead, or if you are feeling tired, take a bath or have a shower.