I took 1500mg paracetamol at once for relieving headache...my weight is 45kg...is that risky in any way or is totally safe?

Yes : not recommended.. Potential harm to the liver.. You may take 500mg every 6 hrs and use ibuprofen 400-600mg every 8 h ass needed

May be ok one time: Maximum one time dose of Paracetamol is 1000 gm and 24 hours not to exceed 4000 mgm Most probably no harm will come if you only did once and you are not drinking alcolhol as well But it may be a better practice to not exceed 1000 mgm as a single dose and 4000 mgm in24 hours And refrain from drinking Alcohol when you are taking Paracetamol

Slight risk but...: Paracetamol in excess doses can cause liver and kidney damage. It is wise to avoid more than 1000 mg at once or more than 3250 mg a day but a single dose of 1500 should not cause significant harm. N-acetyl cysteine (NAC) is a supplement that helps our livers to detoxify paracetamol so if you take it regularly it is a good idea to take NAC (about 500 mg) on the days you take it.