The most common medical abbreviations that you may not know:

Rx*    = Treatment
Hx*    = History
Dx*    = Diagnosis
q*       = Every
qd*     = Every day
qod*   = Every other day
qh*      = Every Hour
S*        = without
SS*      = One & half
C*        = With
SOS*    = If needed
AC*      = Before Meals
PC*       = After meals
BID*      = Twice a Day
TID*       = Thrice a Day
QID       = Four times a day
OD*       = Once a Day
BT*       = Bed Time
hs*         = Bed Time
BBF*     = Before Breakfast
BD*      = Before Dinner
Tw*      = Twice a week
SQ*      = sub cutaneous
IM*       = Intramuscular
ID*       = Intradermal
IV*       = Intravenous
Q4H*     = (every 4 hours)
QOD*    = (every other day)
HS*      = (at bedtime)
PRN*     = (as needed)
PO or "per os"*= (by mouth)
Mg*       = (milligrams)
Mcg/ug*   = (micrograms)
G or Gm*  = (grams)
1TSF* (Teaspoon)  = 5 ml
1 Tablespoonful* =15ml
DDx* =Differential Diagnosis 
Tx* =Treatment 
RTX* =Radiotherapy 
CTx* =Chemotherapy
R/O* =rule out 
s.p* =status post
PMH(x)* =past medical history
Px* =Prognosis 
Ix* =Indication 
CIx* =contraindication 
Bx* =biopsy
Cx* =complication