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Will pepper spray be effective against a mountain lion? Although pepper spray may be an effective deterrent, you really don't want the lion to even get that close. Maintain eye contact if you sight a lion. Lions like to attack from ambush and count on the element of surprise. If the lion sees that you have seen it and are facing it, it is less likely to attack. If you see a lion DON'T RUN! Running stimulates the predator reflex and may cause you to be irresistible to a lion. Don't turn your back. Lions prefer to attack from behind. Turning your back on a lion invites its further interest.
As an expert ambush predator, a lion attack is likely to occur before you are able to effectively and accurately deploy pepper spray, as opposed to a bear charge. Many of the documented lion attacks that involve actual contact describe sudden attacks from behind or above - not a situation where pepper spray is going to be very easy to use, in my opinion, unless a companion can spray the animal as you wrestle/grapple with it.
What to do? Appropriate behavior is described elsewhere – fish and game sites, etc. As much as you may dislike it, make noise as you travel. Most importantly, be aware of your surroundings and avoid areas that facilitate an ambush (thick brush around water, rocky over hangs near dear trails, etc.) Lions hunt the same areas over time and may reuse general sites and tactics if they are successful. Finding an old lion kill site (e.g. a deer skull with bite puncture marks, etc) is a sign you should learn to recognize as well as an obvious fresh kill.
In a close "hand to hand" situation as this is likely to be, I would prefer an actual weapon such as a long blade knife which has multiple uses on your trip. Not a pleasant thought, but a possible reality check. Guns are heavy, often illegal, and useless for any other purpose. Further, seriously injury can occur if you are unfamiliar with one.
I'm thinking about a stun gun/taser myself since these are relatively small & light weight, non-lethal and pretty easy to rapidly use in close quarters and little accuracy is required. It also seems the surprise factor might be effective.