Several versions of a male contraceptive pill or injection are currently being studied. The most recent type utilizes a protein called P-26-h, which is said to neutralize sperm, so it can no longer fertilize an egg. Recently patented in the U-S, this contraceptive will be administered in a vaccine form, and is expected to reach the consumer market as early as 2005. Vaccines last for twelve months, and are reversible once discontinued. Other male pills or injections being developed are based on hormonal methods, similar to female birth control pills. Hormonal contraceptives for men contain progestin, as do female pills, but have the male sex hormone, testosterone, instead of estrogen. They work by lowering sperm counts. Early forms of male hormonal contraceptives caused too many side effects, such as the inability to get an erection, or long-term infertility. But in trials of the new injections, few side effects were reported, other than occasional mood changes or acne. However, much testing remains to be done, and it could be many years before the hormonal contraceptive is available. Male contraception, whether in a daily pill, weekly injection, or yearly vaccine, also faces another challenge: acceptance. For these drugs to be successful, men must be willing to assume the cost and responsibility of taking them as directed.