By Ania Lichtarowicz

BBC health reporter in San Antonio, Texas



A stimulant in many ways

Coffee can do more than just wake you up - it appears that men might also be giving their sperm a kickstart by drinking just a few cups a day.

This, research by a team from Sao Paulo University suggests, increases sperm motility, which could potentially improve the chances of pregnancy.

But caffeine appears to be the only drug that is good for sperm.

Tobacco has no effect on sperm quality but can lead to erectile dysfunction, the same research has found.

Smoking marijuana, though, appears to be bad for sperm and can reduce fertility.

Scientists at Buffalo University found regular smokers had significantly less seminal fluid, and a lower sperm count.

Instead of making sperm slow down and relax, marijuana makes them peak too early: they swim too fast and too soon - they literally burn out before they reach the egg.