I may be a little late replying as it may be sorted now but maybe some advice for if something like this happens again.
Im guessing from the post that he has a history of borrowing money and it going on drugs and obviously we all know that someone who is craving drugs will try anything to get money for them.
I don’t know what stage he is at or if he is currently still using it or if he is recovering etc.
If it is a genuine thing he will probably feel like you’re being hard on him because he could this time be telling the truth. If you’ve said no but he is then coming back trying to put more pressure that could be either because it is genuine and he is hurt that you aren’t willing to help straight away as he wants or due to the fact his cravings are getting worse and he is desperate.
Its such a hard situation to be in as you don’t want to fuel his habit if it is actually for drugs. You don’t want him to suffer if it is the truth and also the worry of what else he might try to do to get money if its for drugs as you suspect.
You’ve done the right thing. Don’t be feel pressured. Think about things logically. What time is it? Is the dentist even open? Can you call the dentist and offer to pay over the phone for the appointment? Can you purchase a ticket online for him to collect at the train station. In this day and age you can do most things remotely so if he genuinely wants your help for his tooth. You couldnexpla6rhe reasons why you don’t want to send him money but will still help and he would be thankful that you’re offering to help.
If he is insistent that he needs the money himself there and then it would lead me to believe its not for the dentist.
I know its hard but tough love is what someone with addiction nesds. Short term they won’t like it but eventually I’m sure he will understand that youre only looking out for his best interest.
I hope that helps in some way and please never feel guilty as its clear you care alot for him and he is lucky to have a sister like you