To short a stock you are basically borrowing stock from your broker, selling that borrowed stock now and hoping the stock goes lower so you can buy it back at a lower price and return it to your broker. Some brokers don't allow this on stocks like this mostly because it might be impossible to find shares that not only can be borrowed for shorting but also might be difficult to return the shares to close the position. The big huge problem is that if the stock goes up you might be given a margin call where the broker decides the stock is going against your short position and feels you might not have enough cash to buy the now higher stock to close the short. In the case of CYDY, if for some reason the SP shoots up your losses could be quite dramatic. I personally can't fathom why anyone would do this with the SP near all-time lows, you can only make money by it going lower and how much lower can it go? Given the near term catalysts that could move the SP up a short position would be frightening! Even those arounf this board that are negative on CYDY for what ever reason must recognize this risk.